Basalts and Phase Diagrams
An Introduction to the Quantitative Use of Phase Diagrams in Igneous Petrology
2nd ed. 2024
Stearns A. Morse
Department of Geosciences, University of Massachusetts, Amherst, MA, USA
ISBN 978-3-030-97880-8 e-ISBN 978-3-030-97882-2 https://doi.org/10.1007/978-3-030-97882-2
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For Dorothy
Preface to the Second Edition
The original edition of Basalts and Phase Diagrams contained, in more than half the book, just the story of how the principles of physical chemistry work for rocks. New and old readers will discover that the same part of the book will look the same as before, because that’s the idea. However, in the Preamble and later parts of this second edition, the news is good: there are wonderful new discoveries and applications that place geochemistry and petrology among the richest of the earth sciences. When it comes to inding natural iceVII in a very deep part of the Earth’s mantle, or reaching out to a one-in-a-million chance of inding a natural example of the most abundant rock in the whole Earth (in a meteorite), you will begin to see how some earth sciences are the most fun. Now we are able, 23 years into a new century, to correctly list ALL the mineral species from the tip of your tongue to the core–mantle boundary. And the real colour of freshly split olivine is not yellow but emerald green. Fortunately, for the challenges of science, there are matters of the inner core that seem not yet quite clear. But not for lack of trying! And it has not been that long since a core–mantle disturbance has been tracked all the way up in a plume to the Western USA. Discoveries in geophysics ramify to our interpretations of the rocks themselves.
Stearns A. Morse Amherst, MA, USA 2023
Preamble: A Remarkable Experiment
The experimental petrologist leads a charmed life: Every experiment yields a result never seen before by man or beast. If she mixes and pressures up a sample every morning, she then opens up a previous day’s (or year’s) sample and sees a unique result. It may be dull or useless, but good or bad, it is unique. And often it is a happy surprise. This note is from an experimentalist’s last experiment, or “run” as we call it.
With a young colleague, Deb Banks, and Professor John Brady as director of the Five College Experimental Laboratory at Smith College, we had inished a study of the thermal history of the 1.307-Ga-old Kiglapait Layered Intrusion in Labrador. This study had reached 14 kbar pressure and 1300°C temperature with highly aluminous minerals augite, orthopyroxene and garnet owing to their unusual compositions to high pressure and temperature. With regard to previous studies, we had extrapolated a mantle source of the parent liquid to occur at 1450°C and 15 kbar before emplacement at 1250°C and 5–6 kbar.
The de initive, multiple-saturated experiments had occurred at about 13 kbar and 1300°C. But I was curious to know what the mineralogy might look like at the liquidus. So I built a inal, off-the-wall experiment with a natural olivine-rich composition. As always, this sample was prepared by grinding the natural rock in a mullite mortar under acetone to maintain a dry condition. Then the micron-scale powder was packed into a hole in graphite to create a high surface energy and easy melting, to exclude any existing macroscopic crystals. The sample was then taken up to a pressure of 13 kbar and an extreme temperature of 1450°C. This high temperature was not expected to survive, and indeed after a small interval, it began to sag.
I watched the temperature closely and wrote it down along with the pressure and time. The temperature sagged at a rate of 1.9°C per minute until it reached a inal temperature at 1375°C, and then remained there. I kept watching for 3.5 h and then quenched the sample, opened it, ground off the graphite and placed the sample in a 1in. epoxy mould overnight. In the morning, I retrieved the sample,
ground and polished it, and held it under mild pressure until the next day.
This was a “unique discovery” day to cap all the others. When examined in the scanning electron microscope and then in the analytical electron microprobe, it showed only one single, enormous, euhedral crystal of olivine (see the Figure). By “enormous”, I mean a rhombic section 60 × 100 microns (micrometres), which is unique in size of all the thousands of experimental crystals I have previously made. Not only is the crystal large, but it is uniform in composition, with classical orthorhombic symmetry. It has three major parallel sides, and at each corner, a tiny growth spurt which signi ies how the crystal grew: the corners show the crystal reaching out for food (as it were) which when gained brings each crystal edge out at constant composition. This is a classic example of how silicate crystals grow from a melt.
But that is not all, by a long shot. The quenched matrix outside the crystal shows local dendrites that look like feathers: they are quench crystals formed during the very fast cooling of the experiment. One corner of the crystal shows rapid external growth in a rectangle. And of greatest interest, the centre of the crystal is a negative crystal form, with edges essentially parallel to those at the outside crystal. Within this pocket is a crystalline product made by the quenching of trapped melt. This pocket, called a melt inclusion, is far more fractionated than the initial bulk composition. Its normative composition includes 68% feldspar, 26% nepheline, a small amount of magnetite, a trace of apatite, and no olivine. The presence of nepheline shows the low fugacity of silica, which has been used to make the olivine.
Melt inclusions can be found in many coarse-grained intrusive rocks, and it is tempting to many observers to infer that these inclusions have trapped a sample of the parent magma. Such an inference must be made with great caution, however. If a melt inclusion can fractionate to a highly evolved composition in 3.5 h, it is not likely to retain a parent liquid composition on a geologic time scale.
The thermal history of this experiment contains a gold-mine of information. What happened during the 75° and 39 min of cooling to 1375°C? Nothing except loss of heat to the surroundings. What then stopped this loss? Crystallization! The experiment is a classic thermal
history of the fractional latent heat of crystallizing magmas. We recognize two types of heat loss in melt systems: the so-called sensible heat that we can feel, and, only when crystals form, the latent heat of crystallization. It turns out that the latent heat that must be removed to allow crystal growth is a very large fraction of the total heat loss of a cooling crystalline system. The so-called fractional latent heat can amount to 80% of the total heat loss, and at the end of crystallization, it is 100% when the last bit of crystal growth stops.
Although the principles of latent heat loss on crystallization have been well-known for ages in the sciences, they have not routinely been invoked in observational petrology until the seminal work of Marian Holness at Cambridge. That work becomes the focus of a chapter on Layered Intrusions in this book.
Further studies of this experiment can be found in the article “Solidi ication of Trapped Liquid in Rocks and Minerals” American Mineralogist 99, pp888–896, 2013.
S. A. Morse
Acknowledgments
My irst encounter with phase equilibria was from my undergraduate professor John Lyons of Dartmouth College, whose introduction to this rigorous science sparked my interest. In due course this led to a joint paper on the Gibbs free energy in garnet-biotite equilibria. Professor Richard Stoiber taught me optical mineralogy and we wrote a book on it. In graduate school at McGill University, visits of Harry Hess and Hat Yoder inspired us particularly. I ran the daily bulletin board and it was my great sorrow to post the death of N. L. Bowen, whom I had hoped to meet. Instead, I discovered Don Lindsley, whose depth of understanding has taught us new tricks and produced wise thoughts over many years. I have been stimulated and edi ied by the remarkable insights of my Carnegie of ice-mate Ikuo Kushiro, who shares the third most references in this book after N. L. Bowen and J. F. Schairer. He and I (among others) share an appreciation of the structure of silicate liquids. Among other supporters who inspire the work of this book have been Gary Ernst, Paul Asimow, Ed Stolper, Lew Ashwal, Dave Stevenson, Ron (the Lord Baron) Oxburgh, Sir Keith Onions, Mark Ghiorso, and Dean Presnall. Many worthy others are cited in an Appendix to this book. Finally, boundless appreciation to John Brady for over two decades of high-pressure collaboration, in petrology and in publication.
1 Introduction
2 Mineralogy and Chemistry of Basalts
3 Crystals and Melts
4 The Phase Rule as a Tool
5 Diopside and Anorthite: Supposedly a Binary Eutectic System
6 Plagioclase: The System An-Ab
7 Diopside-Albite: A Complex System
8 Diopside-Anorthite-Albite
9 Forsterite-Diopside-Anorthite: A Basaltic Analog
10 Incongruent Melting: The System Forsterite-Silica
11 Forsterite-Anorthite-Silica: Incongruent Melting in a Ternary System
12 Forsterite-Diopside-Silica: Pyroxenes and Their Reactions
13 Layered Intrusions
14 Nepheline–Silica and the Rest of the Basalt Tetrahedron
15 Potassium: Petrogeny’s Residua System and Ternary Feldspars
16 Iron and Oxygen
17 Iron-Bearing Olivines and Pyroxenes
18 The Effects of High Pressure
19 Effects of Volatiles at High Pressure
20 Leftovers Appendix Bibliography
© Springer Nature Switzerland AG 2024
S. A. Morse, Basalts and Phase Diagrams
https://doi org/10 1007/978-3-030-97882-2 1
1. Introduction
Stearns
(1)
A. Morse1
Department of Geosciences, University of Massachusetts, Amherst, MA, USA
Basalt Problems
Basalt is the most abundant rock on Earth. It forms the ocean ridges, slides off onto the ocean loor, and eventually dives back down. Basalt is the lava which pours out of Kilauea in Hawaii, and maybe crosses a tar road without melting it (we love to play that ace). Basalt is the rock that formed the island of Surtsey, off Iceland, as people watched. It appears in Japan and, in the US, in both New York and Washington State. The name is applied to the solid rock, the lava, and to the magma which, on eruption, becomes lava.
What do we know of these materials? We may know the composition of the rock, and even of the lava at various stages of eruption, from chemical analyses. Seismic evidence tells us that earthquakes from 50 to 60 km deep precede Hawaiian eruptions; this is far beneath the Mohorovicic discontinuity, and we can reasonably infer that the earthquakes are related to movement of magma in the upper mantle. The source of magma is therefore evidently the mantle. The temperature of the lava at eruption can be measured, and commonly falls in the range 1150–1250°C. We know from physical chemistry that hot solid rock on ascending in the mantle can partially melt to yield basalt. We know that three major types of peridotite (plagioclase, spinel, and garnet) can occur in the mantle and act as the source rocks for basalt magma. It would seem that we know a great deal about basalt.
But many matters remain uncertain about basalt. We do not really know the depth of magma generation in speci ic cases, for the seismic events may only represent the passage of magma. We do not know the exact composition or temperature of generation, since both of these are altered on passage to the surface. We do not know precisely what is melted to form the original magma. We do not often know what crystals form in transit, and of these, which are left behind and which are loated upward. We do not always know what forces impel the magma to the surface, although buoyancy forces would seem most important. We do not know in detail what processes of melting or crystallization may produce the variety of basalt magma compositions which we see. We do not understand the ultimate sources of heat required to generate basalt magma, although we know that some of it must come from the decay of radioactive elements and some from primordial heat left in the deep earth. In particular, we do not understand very well how heat is concentrated within the mantle. We do not even know very well the chemical and physical changes which take place during crystallization at the surface, although studies in Hawaii have greatly improved our knowledge of at least one speci ic batch of magma. In short, we still have only a general idea of the origin of basalt magma, the progenitor of the most common igneous rock type at the surface of the crust. These problems are multiplied further in the case of plutonic (gabbroic) rocks which we can never observe in the process of formation.
What we do know of basalt genesis comes from a wide variety of studies: geochemical, geophysical, and petrological. Of petrological questions, the chief ones are: what rock is melted or partly melted in the mantle to form the relatively uniform magma which we classify as basalt, and what processes can work on such a magma to produce the suites of differentiated rocks which are associated with basalt? The role of experimental petrology has been to place limits on the answers to these questions, to identify parents and processes which are capable of producing what we see, and to shelve those which appear not to work.
There is no single, sharply de ined basalt problem. There is a range of problems having to do, irst, with the source of magma, and second, with what the magma may become. These two threads of basalt study are inextricably entwined, for the question of the fate of magmas may or may not be resolved at the instant of origin. In this book, we shall
keep both melting within the mantle and crystallization of the resulting magmas in view, and we shall ind, as we go, a number of working hypotheses which deserve to be considered concurrently.
The study of basalts can be helpfully framed in terms of the dichotomy between alkali basalt and tholeiite (de ined and discussed in the next chapter). The ocean loor is made of tholeiite, erupted at ocean ridges and carried away from them by lowing mantle beneath. Tholeiite also occurs abundantly in continental lood basalts. It is therefore the most abundant and commonly made rock in the earth’s crust. Alkali basalts occur on many ocean islands and at hotspots, in lesser quantities, but with chemical characteristics that are important clues to the structure and history of the earth’s mantle.
A particular basalt problem lies in the eruption, often from the same vent, of tholeiitic basalts at one time and alkali basalts at another time.
Alkali basalt tends to have a low silica content along with high alkalies. Tholeiite tends to have more silica and lesser alkalies. Tholeiitic basalts most commonly carry interstitial quartz or micropegmatite, or both, when well-crystallized. It is also commonly observed that silicasaturated rocks such as dacite are found associated with tholeiitic basalts. Since the interstitial materials on the one hand and dacitic or rhyolitic rocks on the other hand have much the same composition, a genetic sequence tholeiite → rhyolite may reasonably be inferred. Similarly, alkali basalts never carry interstitial quartz or micropegmatite, but may carry interstitial microsyenite or an undersaturated mineral such as a feldspathoid, commonly nepheline. They are commonly associated in the ield with such silicaundersaturated rocks as trachyte and phonolite. Hence no great intuitive leap is required to deduce the genetic sequence alkali basalt → phonolite.
A genetic sequence might be implemented in a variety of ways; we shall commonly be concerned in this book with both partial melting at the source and later fractional crystallization as competing means of achieving a genetic sequence. The fractional crystallization mechanism is one suggested by the internal evidence of the rocks, for if, as textures indicate, an interstitial residuum (of, say, micropegmatite or microsyenite) represents a late-solidi ied fraction of liquid (i.e. magma), it is but a small step to infer that separation of all the other crystals
from this small fraction of magma would yield an appropriately advanced rock, such as rhyolite or trachyte.
Basalts other than alkali basalt and tholeiite are well known. Among these are high-alumina basalts, which occur in island arcs and continental margins. High-alumina basalts in general appear to be high in the components of plagioclase feldspar, even when glassy, hence apparently not mechanically enriched in plagioclase crystals. There is also a class of basalts intermediate between alkali basalt and tholeiite, as de ined above, which are neither oversaturated nor undersaturated with respect to silica, and which belong to a genetic sequence culminating in trachyte, itself neither oversaturated nor undersaturated. But for present purposes we may treat these other types of basalt as bothersome diversions from a simple model, and illustrate many valid points in terms of arbitrarily de ined alkali basalt and tholeiite.
The implied genetic sequences of these latter two types pose a fundamental question: do the two magma types have a common or diverse origin? If common, is there a common parent magma, or a common parental mantle rock? If diverse, what are the diverse parent magmas or rocks? In this book, we shall not attempt to arrive at a irm decision in favour of a single one of these possibilities, but instead we shall inquire how to evaluate the various propositions by means of phase diagrams.
Other important methods of evaluation, such as studies of minor element distribution, are largely beyond the scope of this book, but in regard to minor elements it should be pointed out that the principles of phase diagrams usually apply to minor as well as major elements. The experimental study of basaltic minerals and their phase relations is, in any event, a fundamental and powerful means of evaluating hypotheses of basalt origin.
Phase Diagrams as Models of Rocks
At one time there was quite an uproar over the application of phase diagrams to natural rocks, some claiming that since the natural rocks are obviously more complicated than synthetic systems, the phase diagrams are simplistic and misleading. We may now cheerfully
relegate the uproar to the history and philosophy of science, and say we are sophisticated enough to have a word which should long ago have disarmed the combatants: phase diagrams are models of natural rocks. We should not hesitate to confess that they are simplistic, but if we are conscious of using them as models, they need not be misleading. As models they can help us to understand the ground rules of the game, the purpose of which is to discover rock-forming processes which are inaccessible to direct view.
In order for phase diagrams to serve as instructive models, they must deal initially with relatively few chemical elements, each of which is present in a state of high purity; in other words, the models with which we work should be chemically simple. If this were not so, we should quickly be presented with experimental results, due to impurities or complexities, which we could not interpret in any straightforward way. This is simply to say that we wish our experiments to be controlled, so that we may change one variable at a time and discover the result of changing that variable. Once a few fundamental principles are discovered, we may make more and more complicated models, till they approach natural rocks themselves.
Experiments on natural rocks may provide even better models than those on synthetic systems, but even these experiments generate models rather than duplicating natural processes, because we cannot a priori de ine, let alone control, all the variables of the natural process. If we could, our work would be done. The interpretation of experiments on natural rocks is risky, and impossible without the principles furnished by synthetic systems in which all the variables may be controlled.
This book treats a limited number of experimental systems, and shows how they may be brought to bear on the problems of basalts. The emphasis is on the application of phase diagrams to both petrologic problems and experimental problems. We shall see that they may be used both qualitatively and quantitatively, as guides to research and guides to rock genesis. Although the theme is basalt genesis, the principles encountered are applicable over the ield of igneous petrology in general.
We shall be concerned, in each system studied, with both crystallization and melting phenomena, since basalt magmas must be
approached both from the standpoint of what they may become (residual liquids produced by fractional crystallization) and from the standpoint of how they originate (partial or complete melting of mantle rocks).
We begin with a brief discussion of basalts, in Chap. 2, followed by discussions of some pertinent physical chemistry in Chaps. 3 and 4. With Chap. 5 there begins a systematic exposition of phase diagrams, their experimental background, and their interpretation. Towards the end of the book, we shall encounter some welcome new treatments of phase equilibria.
© Springer Nature Switzerland AG 2024
S. A. Morse, Basalts and Phase Diagrams
https://doi.org/10.1007/978-3-030-97882-2 2
2. Mineralogy and Chemistry of Basalts
Stearns A. Morse1
(1)
Department of Geosciences, University of Massachusetts, Amherst, MA, USA
Mineralogy
When basaltic lava is abruptly quenched, it solidi ies to glass. When slowly cooled, it crystallizes, more or less completely, to an assemblage of minerals. We may speak of the mineralogy of basalts in terms of these minerals (modal composition), and of the chemistry of basalts both in terms of elements (or oxides) and the potential minerals that might form if the lava were slowly cooled (normative composition). The calculation of normative minerals is discussed below, but irst we may consider the actual minerals found in wellcrystallized basalts.
The two essential minerals of greatest importance are plagioclase and pyroxene, since these make up perhaps 80% of many basalts. Indeed, for a starting model, we should not be far off base to consider plagioclase and pyroxene alone for ideas of basalt crystallization; this is precisely what we shall do in Chaps. 5–8. The plagioclase involved is an intermediate member of the An-Ab series, and Bowen (1928) showed that a composition very near An50 occurs most frequently in basalts. The pyroxene is chie ly calcic, that is, a member of the augite series, usually more magnesian than ferrous-rich (Mg/(Mg + Fe) > 0.5), and commonly not very far from diopside, although it ranges to subcalcic augite. Calcium-poor pyroxene, generally hypersthene, may or may not be present, depending on the abundance of silica relative to other constituents. Ca-poor pyroxene is a critical mineral in basalt classi ication, its presence signifying tholeiitic basalt, and its absence usually implying alkali basalt.
Olivine is another critical mineral in basalt classi ication. In alkali basalt, it is commonly present instead of Ca-poor pyroxene. In olivine tholeiites, it is present in company with Ca-poor pyroxene.
By adding olivine, our basalt model has grown to four minerals: plagioclase, two pyroxenes, and olivine. These four minerals make a model that is indeed a close analogue of natural basalts. The only ubiquitous mineral that we have left out so far is Fe-Ti oxide, usually titaniferous magnetite, perhaps in company with ilmenite, although more oxidized varieties occur. We shall ind that consideration of these non-silicates of cations which can easily be oxidized requires a special type of experimental and theoretical treatment, in which the availability of oxygen must be controlled or speci ied in some way.
Other minerals that occur in basalts, and that play an important role in classi ication when they occur, are quartz and nepheline. Quartz commonly occurs in company with Ca-poor pyroxene. Olivine may occur, but if so it is usually mantled by a more siliceous mineral, commonly Ca-poor pyroxene, for magnesian olivine is not stable in the presence of excess silica, because of the reaction (2.1)
Rocks that carry quartz are silica-oversaturated rocks, and rocks that carry Ca-poor pyroxene instead of quartz could be described as silicasaturated rocks.
Nepheline occurs in some rocks lacking Ca-poor pyroxene; its presence is due to a de iciency of silica required to form feldspar. Were such silica available, nepheline would react to form the albite component of plagioclase by the reaction
(2.2)
The presence of nepheline (or another feldspathoid) therefore clearly signi ies a state of silica-undersaturation, i.e. an amount of silica too small for the continual formation of feldspar throughout the crystallization history of the rocks. Nepheline is accompanied by olivine, this being the most silicapoor Fe-Mg silicate in common basaltic rocks. Nepheline-bearing basalts lie at the opposite end of a spectrum from quartz-bearing basalts; the spectrum consists of the sequence
Basanite (Ne-bearing): critically undersaturated
Alkali basalt: critically undersaturated
Olivine tholeiite: undersaturated
Nepheline basalt: saturated
Tholeiite (Q-bearing): oversaturated
Chemistry
Table 2.1 lists the average chemical composition of crustal rocks, estimated from Rudnick and Gao (2005, Table 10) who give all Fe as FeO. The compositions are expressed as weight, atom, and volume per cent. This table leaves no doubt that the earth’s crust is dominated by oxygen, and that in terms of weight of atoms, silicon runs a strong second. The volume per cent column re lects the small ionic radius of silicon and the large radii of Ca, Na, and K relative to Si.
Table 2.1 Chemical composition of the earth’s crust (oceanic and continental), as estimated from Rudnick and Gao (2005)
As oxides Wt% As atoms
aAfter Whittaker and Muntus (1970); in A (0.001 picometers)
Table 2.2 gives the chemical composition of the average basalt, as estimated from the data of Chayes (1972), and stated in the usual format of weight per cent of oxides. Again, silicon and oxygen dominate the analysis (as SiO2, near 50% in all basalts); Al2O3 is next in abundance, and CaO, MgO, and FeO are closely similar, averaging a little less than 10%. Other oxides are almost always below the 5% level, and until one has inspected many basalt analyses, they all tend to look about the same, despite important differences
that do exist. There is a way of treating chemical analyses, however, in such a way as to bring out important differences that are not manifest on simple inspection. This treatment is the norm calculation.
Table 2.2 Average basalt wt. %
SiO2 47.65
TiO2 2 14
Al2O3 15 28
Fe2O3 3 57
FeO 7 54
MgO 7.52
CaO 9 91
Na
Total 99.77
The CIPW Norm
The norm calculation is an arbitrary formula for casting a chemical analysis of a rock into potential minerals that could form if the rock crystallized completely under idealized conditions. Many different formulas have been proposed, some with a view to enabling further calculations (e.g. the molecular norms of Niggli 1936 and Eskola 1954), and some with a view to representing more exactly the actual mineralogy of the rock (metamorphic norms: see Barth 1962, pp. 337–343). For many purposes, however, the CIPW1 norm is quite adequate,2 and valuable because of the large catalogue of CIPW norms available for comparison, e.g. Washington. The calculation has great utility in discussing ine-grained or glassy rocks whose actual mineralogy is obscure or non-existent, and it provides an important link between natural rocks and experimental systems that are de ined by their chemistry alone.
The CIPW norm is a weight norm, that is, the inal result is expressed as weight per cent of the various normative minerals, or potential minerals implied by the chemical analysis. With common basaltic rocks, these minerals are the feldspars, pyroxenes, olivine, ilmenite, magnetite, apatite, and sometimes nepheline or quartz. The irst three groups, which are solid solutions, are calculated in terms of their end members, so that one may recalculate the normative feldspar composition as percentages of An,3 Ab, and Or, or the normative pyroxene compositions in terms of Wo, En, and Fs, or the normative olivine composition in terms of Fo and Fa (Table 2.3).
Salic group
Quartz (q) SiO2
Orthoclase (or) K2·Al2O3·6SiO2
Albite (ab) Na2O·Al2O3·6SiO2
Anorthite (an) CaO·Al2O3·2SiO2
Leucite (lc) K2O·Al2O3·4SiO2
Nepheline (ne) Na2O·Al2O3·2SiO2
Kalsilite (ks) K2O·Al2O3·2SiO2
Femic group
Diopside (di) CaO·(Mg,Fe)O·2SiO2
Wollastonite (wo) CaO·SiO2
Hypersthene (hy)
Olivine (ol) 2(Mg,Fe)O·SiO2
Forsterite (fo) 2MgO·SiO2
Fayalite (fa) 2FeO·SiO2
Magnetite (mt) FeO·Fe2O3
Ilmenite (il) FeO·TiO2
Apatite (ap) 3(3CaO·P2O5)·CaF2
Table 2.3 Common normative mineral molecules in the CIPW norm
Note: Diopside may be calculated into its wo, en, fs components. Hypersthene, if present, is always calculated in addition to these
One may rant on at some length about the method and purpose of the norm calculation, but no amount of ranting is so instructive as the performance of a few calculations. The calculation is easily done with a pocket calculator, and three or four examples should suf ice to show what the norm calculation is all about.4 A simpli ied calculation procedure, adequate for most basaltic rocks, is given in Appendix I, along with a shorthand form that facilitates the calculation. A single example is given below to suggest the general outline of the calculation, using the Chayes average basalt from Table 2.2.
In order to calculate mineral formulae from a weight per cent analysis, the analysis must irst be recast into molecular proportions (Tables 2.4 and 2.5). This is done by dividing the weight per cent of each oxide by the rounded molecular weight of that oxide. The molecular proportions are then used up in a standard order, irst by assigning all P2O5 to apatite, using an appropriate amount of CaO to yield an arbitrary apatite formula. Then TiO2 is used to form ilmenite, FeO·TiO2, with an equal amount of FeO, and so on. A debit ledger enables one to keep track of the amounts of CaO, FeO, etc. used. The calculation proceeds in routine fashion through hypersthene (Hy), at which point a summation is made of all silica used. If the original molecular proportion of silica is in excess of that used, the rock is oversaturated, and the excess silica is reported as quartz in the norm. If the original amount is less than that used, some or all of the MgO, FeO assigned to hypersthene must be recalculated to the less siliceous mineral olivine. If, when all hypersthene is thus converted to olivine, too much silica has still been used, some or all of the Na2O, Al2O3 assigned to albite must be recalculated to the less siliceous mineral nepheline, and the rock is silica-undersaturated. If, as rarely happens, too much silica has still been used, orthoclase must be converted to leucite. When the appropriate silica balance has been achieved, the molecular amounts for each normative mineral are multiplied by the molecular weight of the mineral to achieve a weight per cent norm. H2O and other volatiles are ignored in the calculation, and simply added on at the end to see if the summation approximates that of the original chemical analysis, which it should if no errors have been made.
Table 2.4 Example of norm calculation, using the average basalt of Table 2 2
Table 2.5 List of normative minerals from the calculation above
a Mol.No. of irst oxide in Col.I × Col.III
A few trial norm calculations should be made by hand in order to appreciate the chemical mineralogy that underlies the norm scheme. It is best to begin with an oversaturated tholeiite, which is the simplest calculation, then proceed to an olivine-hypersthene tholeiite, and inally to a nepheline-normative alkali basalt. There are further teaching and learning approaches available in the exercise procedures of Brady (http://www. science.smith.edu/~jbrady/petrology/igrocks-topics/norms/norms-page01. php).
Historical Note: CIPW
The turn of the century at 1900 marked a peak in the connection between ield and laboratory studies of rocks. By then the United States Geological Survey was already 20 years old with an analytical branch in Colorado. Young scientists were travelling to Germany and bringing back methods and examples of analysis. Among these were C. Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and the bearded Henry Stephens Washington. In and out of the Survey, this group of four generated the CIPW classi ication system that is still in use. There is so much of interest in this process that it helped to generate a history of Petrology in a 686-page book “Mind over Magma” by Davis A. Young (2003, Princeton). The portrait of Washington stood on the wall over my desk at the old Geophysical Laboratory, and there was an old anecdote that he was quoted as saying “If the alkalies are low, smoke another cigar.” Professor Young also edited a newly discovered and wonderful autobiography of Joseph Iddings: “Recollections of a Petrologist.” GSA Special Paper 512 (2015). He also wrote a study of Bowen: “Crystallization and Differentiation: The evolution of a Theory.” (Mineralogical Society of America Monograph Series #4, 1998).
The Basalt Tetrahedron: A Model for Basalt Study
Yoder and Tilley (1962) devised a scheme for classifying basaltic rocks by their CIPW norms, using the concept of a “basalt tetrahedron.” This tetrahedron (Figs. 2.1 and 2.2) is simply the quaternary system forsteritediopside-nepheline-quartz, a group of minerals that serves, to a irst approximation, as a model of basalts. Plagioclase is represented by albite, which lies on the line nepheline (Ne)-quartz (Q) by virtue of reaction (2.2). Pyroxene is represented by diopside, at one corner of the tetrahedron, and enstatite, which lies between forsterite (Fo) and quartz by virtue of reaction (2.1).
Fig. 2.1 The fundamental basalt tetrahedron of Yoder and Tilley (1962), the system Di-FoNe-Qz, showing the plane of silica saturation Di-En-Ab, and the critical plane of silica undersaturation Di-Fo-Ab. Although iron-free, the system accounts for the major phases of basalts
Fig. 2.2 The basalt tetrahedron generalized to admit iron members of Di and Fo corners (i e olivine in general instead of the end member Fo, etc ), and An, and exploded to show nomenclature of major basalt types based on their normative composition
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"I hope we shall. She's out to Ponsworthy with Mrs. Bassett to-day; and the Bassetts are God-fearing people in our own station of life."
"If she was to go there, she'd only be nursemaid to four young children," declared Faith.
"Then, if that's all there is to it, she won't go there," answered Ben.
"And what if nothing to suit your opinions can be found, father?" asked his wife.
"Then—then she'll be forced to stop here, I'm afraid, my dear."
"And what if I said I wouldn't if she did?" flashed out Jane.
"There's some questions beneath answering, Jane, and that's one of them," replied Mr. Bamsey.
In the pause that followed, Mr. Chaffe, who had been smoking in the chimney corner of the house-place where they sat, addressed the family.
Jane, however, did not stop to listen. She began to remove the supper things and came and went.
"Ben's so right as he can be in my opinion, and if you think, you'll see he's right, Faith," said Arthur. "He founds what he says upon the fact that Dinah has done the proper thing to give John up; and if you could only see that, instead of blaming her and thinking hardly of her for so doing, you'd admit she was not to be punished for what she done. We all make mistakes, and though I don't know nothing about love from personal experience, I've seen it working in the world, for good or ill, these fifty years very near. And a tricky thing it is, and Dinah ain't the first that thought she was in love when she wasn't, and won't be the last. There's some would have gone on with it and married Johnny just the same, for one reason and another; but in my humble judgment a girl who can marry a man she doesn't love, for any reason, be little better than a scarlet woman. And when Dinah found there weren't love on both sides, very properly she owned up and said so."
Faith Bamsey listened quietly.
"I've pretty well come to that myself, Arthur," she said. "I may say I go that far now. It was a burning shame, of course, as Dinah couldn't make up her mind months and months ago; but when I tell her that, she says she didn't know her mind. And so not a word against her. She's a saint and worthy of all praise, and I dare say we ought to kiss her feet and bless her. But what next? That's all I humbly want to know? Ben, you see, is very jealous indeed for Dinah; but, on the other side, I like John to be free to come and go from his mother's home; and you won't say that's unnatural. But while she's here, angel though she may be, come John can't; and that's not unnatural either."
She smouldered bitterly under her level speech and self-control.
"All good—all good," declared Mr. Chaffe. "And if I may speak for Ben, I should say he grasps the point as firm as you do, Faith. Dinah's wishful to go; she'd go to-morrow if it was only to be a goose-girl; but that wouldn't be seemly, and you can leave Ben to do his duty in that matter and not let any personal feelings interfere. In fact the more he cared for Dinah, the more he would see she must go out into the world now, for the sake of all parties. The rightful place will be found for her, and I always say that when people do their part up to the point where they may fairly look to Providence to go on with it, then Providence be very quick to take up the running. And if Providence don't, it's because our part have not been done right."
"This very night," said Ben, "Dinah may come back in sight of work at Ponsworthy."
"There remains John," continued Mr. Chaffe, "and John's gone through the fire very brave indeed by all accounts, without a crack, thank God. You've every right to be proud of him; and his turn will come. The Lord tempers the wind to the shorn lamb, and no doubt his future mate will be along in due course for his comfort and uplifting."
Dinah returned a few minutes later and she expressed a desire to go to Ponsworthy; but Mrs. Bamsey's prediction was correct: her work with the
Bassetts must be that of a nurse and no more.
"Providence haven't spoke yet then," said Mr. Bamsey; "but as Arthur very truly says, we've reached a point when we may fairly count to hear a seasonable word afore long; and doubtless we shall do."
Mr. Chaffe presently went home. But for all his smooth speeches, none knew better than he where the fret and difficulty began; and he was aware that it would never end while Dinah remained at Green Hayes.
"If nought's done, in less than no time, she'll make a bolt," he reflected. "She's that sort of woman; and for all us can say, it may be the will of Providence to cut the knot in that manner. I hope not, however, for 'twould be a bitter blow to Ben and fill his old age with sorrow."
He was so impressed with this dark possibility that he decided to see Dinah at the first opportunity and warn her against it.
"A very curious, puzzling thing," thought Mr. Chaffe, "that the price for well-doing be often stiffer than the price for bad. But the good man should keep in mind that the credit side be growing for all he suffers. If we can't trust the recording angel's book-keeping, who should we trust? The wicked may flourish like the green bay; but the end of the green bay be fire, come soon, or come late."
He passed a neighbour in the darkness going home and published his reflection.
"Be that you, Nicholas Gaunter? So it is then. And here, on my way, I was filled with a great thought, Nicholas."
Mr. Gaunter—a hedge tacker of low repute—had drunk too much beer; but not too much not to know it. He concealed his error and Arthur failed to observe the truth.
"A oner for thoughts you be, Mr. Chaffe," he said.
"Yes—they come; and it just flashed over my mind, Nicholas, that goodness breeds life and a good deed can't perish out of the land; but the payment of evil is death—sure and certain."
"Only if you done a murder," said Mr. Gaunter.
"Pass on, Nicholas, pass on. The thought be too deep for your order of mind I'm afraid," replied the older man.
ON ECONOMICS
On a June evening Lawrence Maynard fell in with Dinah at Buckland, near the cottage of the old huntsman. Accident was responsible for their meeting, and they had not seen each other since the girl's engagement was at an end. Now the cowman was on his way to spend an evening with Enoch Withycombe, while Dinah intended to visit Falcon Farm and beg Susan and Mr. Stockman to interest themselves on her account and find her work.
"I can't get anything to do that will pleasure foster-father," she said. "He's so hard to please where I'm concerned, and he don't quite see so plain as I do that it's bad for me and everybody else my biding there."
"I dare say it is."
"Where's your black armlet what you wore for your dead master in Somerset?" she asked.
"I've left it off now with these new clothes."
She nodded.
"I'm going to see Cousin Joe and Susan. She's always been terrible kind to me. So has he for that matter. How have they took this? If they're very much against me, perhaps I'd best not go."
"It's interested 'em a lot. They've heard John's side mostly, because he comes up over now and again. But they keep fairly open minds about it."
"They don't know why I done it?"
"Yes. I ventured to say a word or two. No business of mine; but I just went so far as to explain that I'd seen you last bank holiday, and you told me what you thought to do, and why."
"Thank you I'm sure."
"No call for that. Common fairness. Mr. Stockman's very good to me and lets me talk if I've a mind to. He's a far-sighted, fair man, or so I find him."
"They won't jump down my throat, then?"
"Not likely. I'm going in to have a tell with Mr. Withycombe now. He's poorly, and a neighbour cheers him up and makes him forget his pains."
"What did he think about it, Mr. Maynard?"
"He thought you was right, I believe."
"I'm very glad of that. And what did you think?"
"You know what I thought, miss. I thought you was dead right."
She kept silent for a moment. Then she spoke.
"I wish to God Johnny would see it."
"He will—some day. He don't yet. He——"
Maynard stopped. She put her hand on his arm eagerly.
"Is there anything I could do or say to help him, should you think? If there was, I'd do it if it killed me."
"Nothing much to help him I should reckon."
"What was you going to say when you stopped?"
"Nothing worth saying—at least, something better not spoken for the minute."
She considered this.
"Please tell me if it can help him," she begged.
Now it was his turn to weigh his thoughts and the thing he had on his lips. He decided that he ought to tell her.
"It's this, then. They think at Falcon Farm that, if Johnny is patient, things may yet come right."
Dinah was cast down.
"Oh, I'm sorry they talk like that. Why do they?"
"Because they've seen him and not you, perhaps."
"I don't reckon I'll go now," said Dinah, but continued, before he could advise: "Yes, I will. I must. If there's any feeling like that about, it's only right they should know. I'm not the sort to play with a chap, and it's cruel to let Johnny think I am. But does he?"
"I dare say not."
They talked for another ten minutes. Then she prepared to go up the hill.
"You've done me good," she said, "and I'm very glad I met you. And I'd like to meet you again, please. D'you mind that walk I wanted to go? Will you take it now?"
He hesitated.
"Not if you don't want."
"It's like this, Miss Waycott. If there's a ghost of a chance that you go back to Johnny Bamsey, then I'd rather not, because it wouldn't be vitty and might add to trouble. So if you're in doubt—even a hair's breadth—we'd better wait."
"I'm not in doubt. I wouldn't have given myself all the hateful grief of doing it, nor yet him, if I hadn't made up my mind. I kept my mouth shut so long as there was a shadow of doubt—and long after there was no doubt for that matter. And you can tell 'em so at Falcon Farm, or anywhere."
"Then I'll be very pleased to take a walk any Sunday if you've a mind to."
"Sunday week then, if I don't find work before. I'll meet you—where?"
He considered.
"If 'tis fair and offering a fine afternoon, I'll be—but that's too far. If we're going to Hey Tor Rock, it's a long way for you anyhow."
"How if I was to come to dinner at Falcon Farm first?" she asked, and he approved the suggestion.
"A very good thought, then we can start from there."
"Sure you don't mind?"
"Proud."
They parted then and Dinah, cheered by the incident of this meeting, went on her way.
She liked Maynard, not for himself, but his attitude to life. Yet, had he been other than himself, she had probably not found him interesting. He was always the same—polite and delicate minded. Such qualities in an
elderly man had left her indifferent; but, as she once said to him, the young turn to the young. Maynard was still young enough to understand youth, and it seemed to Dinah that he had understood her very well. She was grateful to him for promising to take the walk. He would be sure to say sensible things and help her. And she wanted to tell him more about her own feelings. Life had unsettled her, and she was learning painful rather than pleasant facts about it. She began dimly to fear there must be more painful than pleasant facts to learn. Several desires struggled with her—the first to see Johnny again and be forgiven and resume a friendly relation, if that were possible. His sustained anger she could not comprehend; and if, as she began to hear, her old lover still hoped to make it up, the puzzle became still greater.
She reached Falcon Farm with two determinations: to talk to Johnny and declare that she could never change again; and to ask herself to dinner, for the sake of the walk with Lawrence Maynard. To make any mystery of the walk had not occurred to her, or him. She did not even think that anybody might put any particular interpretation upon it.
When she reached the farm on the hill, Joe Stockman and Thomas Palk had been for an hour in conversation. It was an evening when in good heart and more than usually amiable, Joe had offered his horseman a "spot of whisky" from his own bottle, and Thomas, accepting it, had cautiously entered upon a little matter for some time in his mind.
Susan sat at the table mending her father's socks, while the men were by the hearth, for the kitchen fire never went out at Falcon Farm and Joe always found it agreeable after sun-down, even in high summer.
Mr. Palk crept to his theme with great strategy. He spoke of the price of commodities in general and the difficulties that confronted even a bachelor with a good home and satisfactory work.
"The thought of a new black coat do make you tremble nowadays," he said.
"Then put the thought away from you, Thomas," advised Mr. Stockman. "I'm often wishful for little comforts myself, as is natural at my time of life;
but I say to myself, 'The times are hard and these ban't days to set an example of selfishness.' The times are lean, Thomas, and we've got to practise the vartue of going without—high and low alike."
"Everybody knows one thing: that everybody else did ought to be working harder," said Susan. "You hear it all round. Where I go, up or down, I always seem to find men loafing about saying the people did ought to be working harder."
"True for you, Soosie-Toosie. I've marked the like. 'Tis all very well for Thomas here to say the prices be cruel; but the question is, 'Why are they?' And I'll tell you for why. Labour says Capital ought to give more; and Capital says Labour ought to work harder; and so they both stand chattering at each other like magpies and saying the country's going to the devil. Whereas, if they'd take a lesson from us of the land and put their backs into it with good will, the sun would soon come from behind the cloud. If each man would mind his own business and not waste his time judging his neighbour and envying him, we'd get a move on. You don't find the professional people grizzling and whining for more money—doctors and lawyers and such like. Nor farmers neither."
"No," said Thomas, "because their job pays and they fetch in the cash and have enough to put by. I'd be so cheerful as them if I could make so much. I'd work like hell pulling mangel if I could get half as much by it as a dentist do pulling teeth. And the great puzzle to me is why for should pulling teeth be worth a fortune and pulling mangel deny me a new Sunday coat?"
"Never heard you to say such a foolish thing afore, Thomas," answered Joe. "My dear man, you voice the whole silly staple of Labour when you say that. And I always thought you was above the masses in your ideas, as we all are to Buckland—or most of us. A thing is only worth what it will fetch, Thomas, and the root of our trouble at this minute is because Labour is forcing Capital to pay it more than it did ought to fetch."
"Labour's worth what it can get," ventured Susan, and her father rebuked her.
"A very wicked thought and I'm sorry you can sink to it," he said. "It's that opinion and a weak Government that's ruining the kingdom. Look at it, Thomas. Here's a man has three pounds a week for doing what an everyday boy of fifteen could do as well. That's false economy to begin with, because that man can't honestly earn three golden pounds in a week. He haven't got the parts to do it. And if millions of men are getting more than they can earn, what's happening?"
"They must have the money to live," said Thomas.
"For the moment they must," admitted his master, "and they're getting it, but where half their time be wasted is in wrangling over keeping it. The fools won't work, because they're afraid of their lives if they do, their wages will come down; and they don't see, so kitten-blind they are, that the very best thing that could happen to them would be that their wages should come down. For what would that mean? It would mean things was returning to their true values, and that a pound was in sight of being worth twenty bob again."
"That's it," answered Thomas. "If three pound be worth only thirty shilling, they must have three pound."
"Listen to me, my son. Would you rather have three pound, worth thirty shilling, or two pound, worth forty? You'd rather have two worth forty; and when Labour sees that two worth forty be better than three worth thirty, then, very like, Labour will set to work to make two worth forty again. That's what their leading men know so well as me; but they're a damned sight too wicked to rub it into the rank and file, because 'twould ease Capital so well as Labour and they've no wish to do a stroke for Capital or the nation at large. They be out for themselves first and last and always. And while the people be so busy fighting for money that they ain't got time to earn it, so long the English sovereign and the world at large will have to wait to come into its own."
"And meantime three pound be worth less than thirty bob; and that's what interests me most for the minute," said Mr. Palk.
"Don't look at it in a small way, Thomas. Don't darken counsel by thinking of number one," urged Joe. "That's what everybody's doing, God forgive 'em. You preach work, in season and out, for at this gait the younger generation will never know what work means. They be hungering to eat without working, and that means starvation for all. Paper's only paper, Thomas, and gold's always gold, till man ceases to think in the pound sterling. So what we want is to get back on to the sure ground of solid gold and establish ourselves again as the nation with the biggest balance at the bank. But us must take these high questions in a high spirit, and not let little things, like a new black coat, blind the sight."
"You speak for Capital, however," murmured Mr. Palk. "I can't withstand 'e, of course, because I haven't been aggicated; but——"
"I speak for Labour quite so much as for Capital," declared Mr. Stockman. "I began life as a labouring boy and I'm a labouring man still, as you can vouch for. I'm only telling Labour, what it don't know and won't learn, that if it worked harder and jawed less, it would be putting money in its pocket. As things are it's a child yowling for the moon."
"Then I suppose I be," said Thomas, "for I was going to put it to you, man to man, that it would be a Godsend to me if you could lift me five bob, or even three."
Soosie-Toosie cast a frightened glance at Mr. Palk and another at her father; but Joe was smiling.
"More money—eh? Now that's a great thought, Thomas—a very great thought. Fancy! And why for, Thomas, if I may ask without making a hole in my manners?"
"For my dead sister's boy," said Mr. Palk. "There's no money, because his father's out of work and I'm very wishful to lend a hand on his account."
"And very creditable to you, Thomas; and how comes it his lawful father's out of work?"
It was at this moment, to the joy of Susan, that Dinah knocked at the door. She leapt up and thankfully brought the visitor back with her.
Mr. Stockman, too, was pleased.
"Company, Thomas," he said. "We'll take this subject up at another time. Don't think I'll forget it. I never forget anything, for though the body's weak, worse luck, the mind is clear. Dinah, I see—and why not? You'll always find friends here, Orphan Dinah."
Thomas emptied his glass and disappeared, while Dinah plunged into the first object of her visit.
"I'm glad you haven't throwed me over for what I've done," she said.
"Far from it," replied Mr. Stockman. "Is Soosie-Toosie the sort that judges, or be I?"
"We're only terrible sorry for all parties, Dinah," said Susan; "and we hope it will come smooth again."
"So do I," answered the younger; "but not the way you mean, Soosie. For it to come smooth is for John to understand I didn't do a wicked thing, only a mistaken thing. And I had to put the mistaken thing right."
She went over old ground and made it clear that none must expect her to go back.
"I hope I'll live to see John happily wedded," she said. "And I never shan't be happy, I reckon, till he is."
"And what about you?" asked Joe. "What's the truth, Dinah?"
She explained that she was not constituted to love.
"I'm like Soosie," she said. "Us be the sort that's happier single." But Miss Stockman laughed.
"You're a good few years too young to tell like that, Dinah. You wait till all this here storm be blowed over and 'tis calm weather in your mind again. You'm born to be married to the right one. If he don't come along, then, with your experience of making a mistake, you never will be married I dare say; but 'tis any odds he will come along I expect."
Dinah, however, shook her head.
"A mistake like what I've made be a very shattering thing," she said. "I wouldn't have the nerve to go into it no more. There's a lot of unmarried women wanted to carry on the work of the world nowadays."
"And always was," declared Joe. "There's plenty of the sensible sort about, like Soosie-Toosie, who know where they stand and be helping on the world very nice indeed. And though some, here and there, may cast a side glance at marriage, it's often because they don't know when they be well off. However, education's opening their eyes a good deal. The deepest minded sort, such as Susan, don't marry; and even them that do wed put it off a good bit because they see in their wisdom it's better to have a certainty to go to than a hope; and better to be the mother of two than ten. I understand these things, I may tell you, and the moment the world gets wise and puts war away for ever, then us won't hear no more from the parsons about breeding, and the populations will go down and prosperity will go up. A time is coming when a man with ten children will be a disgrace and a quiverful a proper laughing-sport."
"I dare say it will," agreed Dinah.
"Yes—the women will see to that. There was a time when a labouring man bred like a rabbit, in hopes that his dutiful childer would keep him out of the workhouse at the end; but that time's past. The poor women begin to see, like the better-most females, that child-bearing ain't the only use for 'em and not the best fun in the world anyhow."
They promised her to remember her need for work, and Joe undertook to see a friend or two at Ashburton who might be able to find it. Then, thanking them very heartily, she asked a question.
"May I come to dinner Sunday week?"
They approved, and Joe hoped by that time he might be able to report progress.
"I've got another reason," she explained. "Mr. Maynard is a very understanding man and he's promised to go for a walk and show me a stone on the moor I'm wishful to see."
Susan was interested.
"Lor, Dinah!" she said.
Mr. Stockman appeared to be buried in thought for a moment.
"Did he ask you, or did you ask him to go for a walk, Orphan Dinah?" he inquired.
"I asked him. I asked him a long time back and he wouldn't go, because he reckoned Johnny wouldn't like it. But I wanted to see the stone, and I wanted to hear Mr. Maynard talk, because he's a very sensible chap and has said several things that did me good. And so I asked him again, and he's got no objection—not now."
"He's a very sensible man as you say," declared Joe, "a more sensible man for his years I haven't met. In fact he's old for his years—for various reasons."
"Would you have any objection, Cousin Joe?" asked Dinah.
He considered.
"No," he decided. "I wish John could have been of the party, I'm sure; but since that's off for all time, then there's nothing wrong in your taking a walk with Maynard. Nor would there be any harm in any case. I know all about Maynard. He's all right; and, of course, if you asked him to go for a walk, Dinah, he couldn't very well refuse to do so."
"He's a very seeing man," said Dinah, "and he thinks a lot of you, Cousin Joe."
"And why not?"
"He might marry himself," said Susan.
"He's not the sort to hurry it," answered the girl. "He don't care for women overmuch seemingly."
Dinah drank a cup of milk and presently set out to walk home. Susan admired her courage.
"Nothing daunts you," she said. "I wouldn't go down through the woods in the night by myself for the world."
"Night's got no more to it than day," declared the other. "I like it— specially when you have such a lot of trouble on your mind."
She met Maynard returning home, but did not stop more than a moment.
"I'm coming Sunday week," she said, "and Cousin Joe's got no objection to us going out walking."
"Good night, miss. I hope we'll have a fine day for it. Can't go else," he answered.
"How's Mr. Withycombe?"
"Suffering a good bit I'm sorry to say."
"I'm sorry, too."
Lawrence had forgotten the question of the walk while with the old huntsman. Now he considered it and was glad that Dinah had spoken about it in her open fashion. He apprehended pleasure from it, yet doubted a little. There hung a shadow over his reflections—something to which he could not have set a word. In so much that the shade should hover over his own
thoughts it amused him, and assured that it could not cloud Dinah's, he dismissed the futility from his mind.
THE FACE ON THE ROCK
The day came for Dinah's walk with Lawrence Maynard, and though the sky lowered at dawn, before noon the wind had travelled north of west and there was no longer any fear of rain.
They set out, climbed the Beacon and advanced by those rolling stretches of heath and stone that extend to the north of it.
John Bamsey had been to see the Stockmans, and it seemed that his mother, or sister, had now made it plain to him that Dinah would never change her mind.
"He's taking it ill," said Lawrence. "He's not standing up against what he's got to suffer in a very good spirit."
"Us must pray that the right one will come along," answered Dinah.
They talked but little on their way, reached the White Gate, held to the winding road awhile, then returned to the moors and presently stood looking down into the deserted quarries of Hey Tor.
"I'll show you the face on the rock when we turn," he said. "I wanted for you to see this first. A very interesting place and known to me since I was a boy."
Thus he opened a measure of the confidence he designed for her. All the truth about himself he did not propose to tell; but there were things that he
could trust to her; and he meant to do so. His purpose was vague and sprang from no deep emotion. He thought only to distract her mind, perhaps amuse her, and for a time arrest the melancholy flow of her thinking. For she was not cheerful and as yet no outlet for her life and energies had been discovered. Benjamin Bamsey proved obdurate in the matter of her future, and there was come a new and painful element into the life at Green Hayes.
They sat and looked into the quarry. The weathered place was hung with ferns and heath. Deep, green pools lay in the bottom of it and a ring-ousel sat and sang his elfin song, perched on a rusty fragment of iron, driven into the granite by men long since in their graves.
"This was my playground and a place of magic to me when I was a child," said Lawrence, to the surprise of the listener.
"I thought you was a foreigner," she said.
"No. But let everybody else go on thinking so, please. I want it a secret, though it's of little consequence really. I was born a mile from here. The cottage where I lived with my family is a ruin now—I'll show it to you— and me and a little sister used to play on the heath and make our games. They're all gone except that sister. She married and went to Australia. The rest are dead."
"You'm a lonely man then?"
"Used to it. It's only my childhood that the face on the rock comes into, and this deserted quarry. I met a gentleman here once, who told me all about the place. He knew its history and cared for such things. And his talk put great thoughts in my head, for I was thirteen by then and full of ideas already. I got 'em from my mother. She was better bred and born than father and wishful to see me higher than a labourer some day."
Dinah threw herself into his narrative.
"To think of that," she said. "How terrible interesting everybody is, the moment you begin to know the least bit about 'em!"
"I suppose they are. Not that there's anything interesting in me. Only I often catch myself turning back to when I was a boy. The gentleman told me that a lot of the stone cut out from this place is in London now. London Bridge be made of it, and part of the British Museum too. And I never forgot that. I envied those stones, because it seemed to me it would be better to be a bit of London Bridge than what I was."
"What a queer thought," murmured Dinah.
"'Tis a queer thought, but true, that there's plenty of dead stones doing better work in the world than plenty of live men. I used to dream like that when I was a nipper, but I soon had to earn my living, and then there was an end of dreams. Poor folk haven't got no time to dream."
"And not much to dream about most times."
"Plenty to dream about," he assured her, "but we pay our leaders to do the dreaming for us; then, when they've fixed up the dream, they come to us to turn the dreams into reality."
"You'd like to be doing something better than milking cows perhaps?"
"No, I shouldn't—not now. I had ideas, but life knocked 'em out of me."
"Not at your age, I'm sure. You talk as if you was old."
"The heart knows its own bitterness, and a head like mine knows its own weakness," said Lawrence. "If things had gone as I expected, I should never have thought of large questions, and been quite content with the business of running my own life. But things happened to change my outlook and make me think. Then I found I'd got a poor set of brains. I'd just got brains enough to know I was a long way nearer a fool than lots of other men; and I'd just got eyes to see the gulf between. And yet to wish you'd got more brains is only a fool's wish, come to think of it, for the pattern of a man's brain ain't of his own choosing. I suppose nobody's satisfied with what he's got."
"You must be a pretty clever sort of chap to think such things at all," answered Dinah. "And you're a good man, and most times the good ones ain't the right down clever ones. You can't help seeing that."
"For a long time, owing to one thing and another, I was a chap overcome by life," confessed Maynard. "Things fell out that properly dazed me; and it was not till then I began to see the real meaning of life at all. It's much the same with John Bamsey at this minute. While all went smooth, he never saw much beyond the point of his own nose, and never wanted to; then came trouble, and we'll hope it will make his mind bigger when the smart dies. For trouble's no use if it don't do that. Anyway life made me take larger views for a bit. A storm clears the air. Then with time, I settled down again, same as I am now."
"Contented?"
"As near content as I'm ever likely to get. I've simplified my life to the limits. I said to myself, 'Since you can't have what you wanted, have nothing.' And I have nothing."
"That cuts both ways, I reckon," declared Dinah; "you escape a lot of bother, but you lose a good few things that make life better, don't you?"
"To cut a loss is a very wise deed," he answered. "So it seemed to me anyway. That may be wrong, too, in some cases; but if you've got no choice, then you must. Now let me show you where I was born, if you're not tired."
Presently, in the valley far beneath these downs, where the hillside fell to the north and a stream ran in the bottom of a woody coomb, Maynard pointed to a little building. It stood where the land began to ascend again and climb to those rugged piles of granite known as Hound Tor Rocks.
"D'you see that ruin alongside the green croft beside the edge of the woods? That was a fair-sized cottage twenty years ago. My father worked at Hedge Barton, near by, and we lived there till he died. Then we scattered."
Dinah regarded the spot with interest.
"To think of that," she said.
"My playmate was my sister Milly," continued Lawrence. "We were the eldest, and after us came two girls, who both died. Then my mother was with child again, and that brings me to the face on the rock, what you want to hear about."
Dinah, as her custom was, had flung herself entirely into these interests of another being. She had an instinct to do this: it was no art, but a natural impulse in her. At this moment nothing on earth seemed more important and desirable to know than these passages from the boyhood of Lawrence Maynard.
"Such things bring you home to my mind," she said. "Now I'll have a better idea about you; and then you'll be more interesting."
He laughed at that.
"Not very interesting, even to myself, so it's sure I can't be to anybody else," he answered. "Now we'll take Hey Tor Rock on our way back. It'll throw a bit of light on one or two things you've asked me."
They approached the granite bosses of the tor and stood presently beside it, where high on the cliff above them a face bulked enormous and stared into the eye of the westering sun.
The chisels of Nature carve slowly on granite, but once a masterpiece has been wrought, it will outlast many generations of mankind. Such things chance out of slow mouldings, or by sudden strokes. They may be the work of centuries, or the inspiration of a moment—plastic, moulded by patient Time, as the artist models his clay, or glyphic—struck with a blow of lightning, or earthquake, from the stone.
The great rock idols come and go, and haunt lonely cliffs, crown lonely heights, gaze out upon the surges of lonely seas. To Nature these whimsical figures, near enough to man to challenge him, are but faces in the fire, peeping to-day from the flux, and cinders again to-morrow; but, to the short-lived thing they imitate, they endure, while his own generations lapse.
