A Treatise on Rocks, Rock-Weathering, and Soils

Front matter

EOCKS, BOCK-WEATHERING, AND SOILS PLATE 1. — Stone Mountain, Georgia. A residual boss of granite. A TREATISE ON ROCKS, ROCK-WEATHERING AND SOILS BY GEORGE P. MERRILL Curator of Geology in the United States National Museum, and Professor of Geology in the Corcoran Scientific School and Graduate School of Columbian University, "Washington, D.C. Author of "Stones for Building and Decoration," etc. Neto gatfe THE MACMILLAN COMPANY LONDON: MACMILLAN & CO., Ltd. 1897 All rights reserved COPTEIGHT, 1897, By THE MACMILLAN COMPANY. Nortoooft IPrtBS J. S. Cushing & Co. —Berwick St Smith Norwood Mass. U.S.A. "The ruins of an older world are visible in the present structure of our planet ; and the strata which now compose our continents have been once beneath the sea, and were formed out of the waste of pre-existing continents.

The same forces are still destroying, by chemical decomposition or mechanical violence, even the hard est rocks, and transporting the materials to the sea, where they are spread out, and form strata analogous to those of more ancient date." — Hctton. PREFATOEY NOTE In the work here presented the writer has endeavored to bring together in systematic form the results of several years' study of the phenomena attendant upon rock degeneration and soil formation. Although beginning with a discussion of rocks and rock-forming minerals, the work must be con sidered in no sense a petrology as this word is commonly used. What is here given relative to the origin, structure, and composition of rock masses is regarded as an essential introduction to the chapters on rock-weathering. The por tion dealing with the structure and composition of the resultant materials is an essential corollary to these same chapters.

It is believed that no apology is necessary even in this day of many books for bringing out the present work. The origin, structure, and mineral composition of rocks, particularly the eruptive varieties, are matters which have of late received much attention. In fact, it is to these rocks that the petrologists have devoted their best efforts. Since the introduction of the microscope into petrographic work, there has, however, been very little time devoted to the study of rocks in a weathered condition. The chemists have made analyses, but have disre garded the physical and mineralogical nature of the material analyzed. Other workers have studied the physical properties of rocks decayed, — in the form of soils, — but have in their turn disregarded their mineral and chemical nature. The writer has aimed to bring together here such results obtained by these workers in divers fields as it is believed will be for the mutual benefit of all concerned.

Part I The Constituents, Physical And Chemical Properties, And Mode Op Occurrence

OF ROCKS I. INTRODUCTORY A rock is a mineral aggregate ; more than this, it is an essential portion of the earth's crust, a geological body occupying a more or less well-defined position in the structure of the earth, either in the form of stratified beds, eruptive masses, sheets or dikes, or in that of veins and other chemical deposits of. comparatively little importance as regards size and extent. In giving this definition, origin, chemical composition, and state of aggregation of the individual particles are for the time ignored. From a strictly geological standpoint, the beds of loose sand, and even the water of the ocean itself, may be considered as rocks, and either, under favorable circumstances, may undergo a process of induration such as shall be produc tive of the condition of solidity commonly ascribed to rocks by the popular mind.

In ever-varying conditions as regards compactness, color, texture, and structure, rocks form the entire mass of the globe so far as it is as yet made known to us, with the exception of a scarcely appreciable proportion of organic matter. It is rock which forms the substance of mountain ranges and the vast stretches of valley and plain. It is from the rocks that we gain our food, our fuel, and the supplies of metal which are seemingly so essential to our well-being ; we cannot ignore 1 B INTRODUCTORY them, even if we would. We borrow from the rocks that which is essential to our life to-day, but when that brief day is ended return it once more, with neither loss nor gain, to its original source. Those portions of the earth's crust which are available for study comprise at best but a few thousand vertical feet, though from the fact that the stratified rocks have been so extensively thrown out of their original, horizontal position, and again eroded, we are enabled to measure their thickness, and may hence claim to know with a reasonable degree of accuracy the character of the material forming this crust down to a depth of perhaps twenty miles.1 Throughout all this vast thickness, comprising millions upon millions of cubic feet, in weight far beyond all comprehension, we find a constant recurrence of materials alike in composition and similarity in origin to those upon the immediate surface.

There is at times, as noted later, a difference in structure due to metamorphism, between the older, deeper lying portions and those more recent, but the ultimate composition is essentially the same, and all the know ledge thus far gained points to a wonderful unity in nature's methods, and shows with seeming conclusiveness that the geo logical agencies of the past, the methods by which rocks were made and again destroyed, differed in no essential particular from those in progress to-day. What these processes were, how they operated, and with what results, it shall be our aim to here set forth. Among the many interesting, and at first thought seemingly unaccountable, things we shall encounter in the progress of our work, not the least is the fact that so large a proportion of natural objects are more or less out of harmony with their surroundings.

Throughout life every organic being is in a constant struggle with the elements to preserve that life, fulfil all its functions, and gratify its natural desires. No sooner does life depart than decomposition and disintegration ensue. As with organic beings, so with inorganic substances. Every mass of rock pushed up by the faulting and foldinc of the earth's crust, exposed by denudation, or erupted as molten matter from the earth's interior, finds almost at once that its various elements, in their existing combinations, are not in har- 1 The total mean depth of the fossiliferous formations of Europe as stated bv Geikie (Text-book of Geology, p. 675) has been set down as 75,000 feet. INTRODUCTORY 3 mony with their environment. The summer's heat and winter's cold, the chemical action of atmospheres and acidulated rains, combine their forces ; a breaking up ensues, to be succeeded by new combinations and perhaps reconsolidations more in keeping with the then existing circumstances.

An intermediate product in all this endless cycle of change, of disintegration and recombination, is a comparatively thin, superficial mantle of loose debris, which, mixed with more or less organic matter, nearly everywhere covers the land, and by its combined chemical and mechanical properties furnishes food and foothold for myriads of plants, and hence, indirectly, sustenance for man and beast as well. In brief, what is commonly known as soil is but disintegrated and more or less decomposed rock material, inter mingled, perhaps, with organic matter from plant decay. Such being the case, a study of the processes of rock weathering and the transportation, deposition, and physical properties of the resultant debris, is but a study of the origin of soils on the broadest and most comprehensive basis, and soils themselves may justly be regarded as secondary rocks in a state of in complete consolidation.

Their study belongs, therefore, as legitimately to the realm of geology as does that of any sub ject relating to rock formation or other phases of the earth's history. Accepting the above, we will begin our studies by a consideration of (1) the elements which in their single or combined state make up the minerals ; (2) the minerals which make up the rocks ; (3) the rocks themselves, with particular reference to their mineralogical and chemical natures ; (1) the breaking down or degeneration of rocks through processes in part chemi cal and in part mechanical ; and (5) the result of this clasmatic process as manifested in the production of clay, sand, gravel, and incidental soil. There are other points which will be touched upon more briefly, in order to make our work systematic, as the action of wind and water in assorting and redeposit ing rock debris and tending to reduce the land surface to one general level.

THE CHEMICAL ELEMENTS CONSTITUTING ROCKS Although there are 69 elements now known, but 16 occur in any abundance or form more than an extremely small proportion of the material of the earth's crust. Indeed, of this number probably fully one-half, taken collectively, will not constitute more than 4 or 5% of the earth's crust so far as known. These 16, arranged according to their chemical properties and order of their abundance, are as follows : oxygen, silicon, carbon, sulphur, hydrogen, chlorine, phosphorus, fluorine, aluminum, calcium, magnesium, potassium, sodium, iron, manganese, and barium. The eight more important, with their approximate percentage amounts as given by Roscoe and Schorlemmer,1 are as below Oxygen 44.0 to 48.7% Silicon 22.8 to 36.2 Aluminum 9.9 to 6.1 Iron 9.9 to 2.4 Calcium 6.6 to 0.9 Magnesium 2.7 to 0.1 Sodium 2.4 to 2.5 Potassium 1.7 to 3.1 It must not for a moment be imagined, however, that these elements exist for the most part in a free or uncornbined state: on the contrary, in the majority of cases so great is their affinity for one another that it is only momentarily, or under abnormal conditions, that they are met with at all in this elementary form.

Those elements which are most common in the free state, though even these occur more commonly combined with others, are, (1) the gas oxygen, and (2) the solids, carbon, sulphur, and, more rarely, iron. Still more rarehy, and under such abnormal conditions, as exist during volcanic eruptions, are found the free gases, hydrogen, chlorine, and fluorine. The gas nitro gen, although so abundant a constituent of the atmosphere, 1 Treatise on Chemistry, Vol. I, p. 55, 1878. OXYGEN 5 is, as a primary constituent of the earth's crust, almost wholly unknown, and needs no consideration at this stage of our work. Oxygen, as is well known, is the active, even the aggressive, principle of the atmosphere, of which it constitutes about one fifth by bulk. Combined with other elements, it is, however, of vastly greater geological importance, being estimated, as noted above, to constitute from 44 to 48.7% of the entire mass of the earth's crust ; that is to say, could the earth's crust be once more resolved into its original elements, the oxygen thus liberated would be found very nearly equal to all the other elements taken together.

The simpler forms of oxygen com pounds are known as oxides, and of these the oxide of hydrogen, water (H20), is by far the most common, and, anomalous as it may at first seem, is a true mineral and to be classed as an anhydrous oxide at that. Aside from being so essential to human life, oxygen, as will be noted later, is a very potent factor in the manifold changes which are constantly taking place in the more superficial portions of the earth's crust. Silicon. — Next to oxygen silicon is the most abundant of the earth's constituents, though it exists only in combination, either as an oxide (Si02), or with other elements to form silicates. In these two forms it is the predominating con stituent in all but the calcareous rocks. As silica (Si02), or quartz, it forms one of the most indestructible of natural com pounds, and hence is to be found as the prevailing constituent in nearly all sands and soils.

Aluminum is next to oxygen and silicon probably the most important element when regarded from our present standpoint. It occurs mainly in combination with silicon and oxygen, forming an important series of minerals known as aluminous silicates. As a sesquioxide it is well known in the minerals corundum and beauxite. Iron, although less abundant than either oxygen or silica, occupies a very important place as a rock constituent, owing to the variety of compounds of which it forms a part, as well as to the decided colors which are characteristic of its oxides and of the iron-bearing silicates. The most conspicuous forms of iron on the immediate surface of the earth are the oxides, but which at greater depths, or where the atmosphere has as yet exercised less influence, give way to carbonates, sulphides, and silicates.

CHEMICAL ELEMENTS CONSTITUTING THE ROCKS 6 Iron, although so common in combination with other ele ments, occurs but rarely free, owing to its affinity for oxygen. It is possible that far below the surface, beyond the reach of meteoric waters and atmospheric air, it is to be found in a metallic state much more abundantly, but of this we have no other proof than that the specific gravity of the globe, in its entirety, is much greater than that of the most dense minerals which constitute its outer portion. The inference seems un avoidable that at great depths some of these elements exist uncombined, and in a state of greater molecular density than at the surface. Calcium is a very important element of the earth's crust, although, as we have seen, it has been estimated to compose only about one-sixteenth of its mass.

Its most conspicuous form of occurrence is in combination with carbon dioxide, forming the mineral calcite (CaC03), or the rock limestone. In this form it is slightly soluble in water containing carbonic acid, and hence has become an almost universal ingredient of all natural waters, whence it furnishes the lime necessary for the formation of shells and skeletons of the various tribes of mollusca and corals. In combination with sulphuric acid, calcium forms the rock gypsum. It is also an important con stituent of many silicates. Magnesium is found in combination with carbonic acid as carbonate, forming thus au essential part of the rock dolomite. The bitter taste of sea-water and some mineral waters is due to the presence of salts of magnesia. In combination with silica as a silicate it forms an essential part of such rocks as serpentine, soapstone, and talc.

Potassium combined with silica is also an important element in many mineral silicates, as orthoclase, leucite, and nepheline. In smaller amounts it is found in silicates of the mica, amphibole, and pyroxene groups. The following table will serve to show the varying amounts of potash (K20) in rocks of various kinds : — Granite 2.6 to 6.50% Diorite 0.1 to 2.42% Basalt 0.058to 0.50% Gabbro 0.00 to 0.93% Limestone 0.19 to 1.22°/ Sandstone 0.00 to 3.80% Slate (fissile argillite) 0.00 to3.83»/° SODIUM As a chloride, potassium is invariably present in sea-water, and as a nitrate it forms the rare, but valuable mineral nitre, or saltpetre. Sodium. — The most common and wide-spread form of the element sodium is the compound with chlorine known as sodium chloride (NaCl) or common salt. In this form it is the most abundant of the salts occurring in sea-water, and constitutes also rock masses of no inconsiderable dimensions interstratified with other rocks of the earth's crust.

Combined with silica, lime, and alumina, sodium is an important constituent of the soda-lime feldspars, and of numerous other silicate minerals. In the form of carbonate and sulphate it occurs as an incrustation on the surface, or disseminated throughout the soils in poorly drained portions of arid countries, giving rise to the so-called "alkali soils," for which such regions are frequently noted. As a nitrate, sodium occurs in the desert regions of Chili, forming the soda nitre so valuable for fertilizing purposes. Manganese is, next to iron, the most abundant of the heavy metals, occurring as oxide, carbonate, or in combination with two or more other elements as a silicate. Barium is found mainly combined with sulphuric acid, to form the mineral barite or heavy spar. It sometimes occurs as a carbonate, and more rarely as a silicate.

Phosphorus, although existing in comparatively insignificant proportions, is nevertheless an important element, though in nature it occurs only in combination with various bases, principally lime, to form phosphates. In this form it is found in the bones of animals, the seeds of plants, and constitutes the essential portions of the minerals apatite and phosphorite. Though small in proportion, phosphorus is a very important constituent of any fertile soils. Its chief source, in the older, crystalline rocks, is the mineral apatite, as noted later. As found in the secondary rocks, as limestones and marls, it is evidently derived from animal remains. (Seep. 151.) Analy ses have shown that the amount of phosphorus, in the form of phosphoric anhydride (P205), in rocks rarely exceeds 1%, and usually falls much lower, being most abundant in the basic eruptive rocks, as diorites and gabbros, and most lacking in the siliceous fragmentals, as sandstones and slates.

The following table will serve to show the small percentages of this constituent in rocks of various kinds : — CHEMICAL ELEMENTS CONSTITUTING THE ROCKS Granite 0.07 to 0.25% Diorite 0.18 to 1.06 % Basalt 0.03 to 1.18% Limestone 0.06 to 10.00% Shale 0.02 to 0.25% Sandstone 0.00 to 0.1 % Of the solid elements occurring free, or uncombined, carbon is by far the more abundant, being found in the forms known as diamond and graphite, or when quite impure as coal. In combination as a dioxide (C02), it forms the well-known car bonic acid gas, which, like oxygen, is a powerful agent in bringing about important changes in the rocks with which it comes in contact. Free sulphur occurs more rarely, being as a rule a product of volcanic activity, or due to the reduction of the sulphides and sulphates of the metal with which it more commonly exists in combination.

THE MINERALS CONSTITUTING ROCKS A rock, as previously stated, is a mineral aggregate. As a rule, the number of mineral species constituting any essential portion of a rock is very small, seldom exceeding three or four. In common crystalline limestones, the only essential constitu ent is the mineral calcite ; granite, on the other hand, is, as a rule, composed of minerals of three or four independent species. As has been elsewhere stated, the mineral composition of rocks in general is greatly simplified by the wide range of conditions, under which the commonest minerals can be formed, thus allowing their presence in rocks of all classes and of what ever origin. Thus quartz, feldspar, mica, the minerals of the hornblende or pyroxene group, can be formed in a mass cooling from a state of fusion ; they may be crystallized from solution, or be formed from volatilized products.

They are therefore the commonest of minerals and rarely excluded from rocks of any class, since there is no process of rock formation which determines their absence. Moreover, most of the common minerals, like the feldspars, micas, hornblendes, pyroxenes, and the alkaline carbonates, possess the capacity of adapting them selves to a very considerable range of compositions. In the feldspars, for example, the alkalies, lime, soda, or potash may replace each other almost indefinitely, and it is now commonly assumed that true species do not exist, all being but isomorphous admixtures passing into one another by all gradations, and the names albite, oligloclase, anorthite, etc., are to be used only as indicating convenient stopping and starting points in the series. Hornblende or pyroxene, further, may be pure silicates of lime and magnesia, or iron and manganese may partially replace these substances.

Lime carbonate may be pure, or magnesia may replace the lime in any proportion. These illustrations are sufficient to indicate the reason of the great simplicity of rock masses as regards their chief constituents, and that whatever may be the composition of a mass within nature's limits, and whatever may be the conditions of its origin, the .probabilities are that it will be formed essentially of one or more of a half a dozen minerals in some of their varieties. But however great the adaptability of these few minerals may be, they are, nevertheless, subject to very definite laws of chemical equivalence. There are elements which they cannot take into their composition, and there are circumstances which retard their formation while other minerals may be crystallizing. In a mass of more or less accidental composition it may, there fore, be expected that other minerals will form in considerable numbers, but minute quantities.

It is customary to speak of those minerals which form the chief ingredients of any rock, and which may be regarded as characteristic of any particular variety, as the essential constituents, while those which occur in but small quantities, and whose presence or absence does not fundamentally affect its character, are called accessory constituents. The accessory mineral which predomi nates, and which is, as a rule, present in such quantities as to be recognizable by the unaided eye, is the characterizing acces sory. Thus a biotite granite is a stone composed of the essential minerals quartz and potash feldspar, but in which the accessory mineral biotite occurs in such quantities as to give a definite character to the rock. The minerals of rocks may also be con veniently divided into two groups, according as they are products of the first consolidation of the mass or of subsequent changes.

This is the system here adopted. We thus have: — (1) The original or primary constituents, those which formed upon its first consolidation. All the essential constituents are original, but, on the other hand, all the original constituents are not essential. Thus, in granite, quartz and orthoclase are both original and essential, while beryl and zircon or apatite, though original, are not essential. (2) The secondary constituents are those which result from changes in a rock subsequent to its first consolidation, changes which are due in great part to the chemical action of percolating water. Such are the calcite, chalcedony, quartz, and zeo lite deposits which form in the druses and amygdaloidal cavities of traps and other rocks. Below is given a list of the more important rock-forming minerals, arranged as above indicated.

Although these are sufficiently described as regards their chemical and crj'stallo- ROCK-FORMING MINERALS graphic properties in any of the mineralogies, it has seemed advisable to devote some space here to a reconsideration of those most prominent as rock constituents, in order that the individual characteristics of the rocks of which they form a part may be better understood. In passing them in review we will also note briefly the characteristic alteration and de composition products to which they give rise, though the cause of such changes must be left for another chapter. A. Original Minerals. 1. Quartz. 6 a. Muscovite. 6 b. Biotite. 2. The Feldspars. 2 a. Orthoclase. 2 b. Microcline. 2 c. Albite. 2 d. Oligoclase. 2 e. Andesite. 2f. Labradorite, 2 g. Bytownite. 2 h. Anorthite. 6 c. Phlogopite. 7.

Calcite (and Aragonite). 8. Dolomite. 9. Gypsum. 10. Olivine. 11. Garnet. 12. Epidote. 13. Zoisite. 3. The Amphiboles. 3 a. Hornblende. 3 b. Tremolite. 3 c. Actinolite. 3 d. Arvedsonite. 3 e. Glaucophane. 3/ Smaragdite. 14. Andalusite. 15. Staurolite. 16. Scapolite. 17. Elæolite and Nepheline. 18. Leucite. 19. Sodalite. 20. Hauyn (nosean). 21. Apatite. 4. The Monoclinic Pyroxenes. 4 st. Malacolite. 4 b. Diallage. 4 c. Augite. 4 d. Acmite. 4 e. Ægerite. 22. Menaccanite. 23. 'Magnetite. 24. Hematite. 25. Chromite. 26. Halite (common salt). 5. The Rhombic Pyroxenes. 5ct. Enstatite (Bronzite). 5 b. Hypersthene. 27. Fluorite. 28. Graphite. 29. Carbon. 30. Pyrite. 6. The Micas. B. Seconuaky Minerals. 16. Opal. 1. Quartz. 1 c. Tridymite. 1 a. Chalcedony. 16. The Zeolites. 16 a. Pectolite. 16 b. Laumontite. 16 c. Phrenite. 16 ...

Thomsonite. 16 e. _. atrolite. 16/. Analcite. 16 g. Datolite. 16 h. Cbabazite. 16 i. Stilbite. 16 k. He ulandite. 16 1. Pbillipsite. 16 m. Ptilolite. 16 n. Mordenite. 16 o. Harmotome. 17. Hematite. 18. Limonite. 19. Gothite. 20. Turgite. 21. Pyrite. 22. Marcasite. ca, Si02; specific gravity 2.6; 2. Albite. 3. The Amphiboles. 3 a. Hornblende. 3 b. Tremolite. 3 c. Actinolite. 3 d. Uralite. 4. Muscovite (Sericite). 5. The Chlorites. 5 a. ijefferisite. 5 b. Ripidolite. 5 c. Penninite. 5 d. Prochlorite. 6. Calcite (and aragonite). 7. Wollastonite. 8. Scapolite. 9. Garnet. 10. Epidote. 11. Zoisite. 12. Serpentine. 13. Talc. 14. Glauconite. 15. Kaolin. Quartz. — Composition : hardness, 7.1 Pure silica. This is one of the and most widely distributed commonest minerals of the earth's crust, and forms an essential constituent in a variety of eruptive and sedimentary rocks, such as granite, 1 For convenience in determining minerals, the "scale of hardness" given below has been adopted by mineralogists.

By means of it one is enabled to designate the comparative hardness of minerals with ease and definiteness. Thus, in saying that serpentine has a hardness equal to 4, is meant that it is of the same hardness as the mineral fluorite , and can therefore be cut with a knife, hut less readily than calcite or marble. 1. Talc : Easily scratched with the thumbnail. 2. Gypsum : Can be scratched by the thumbnail. 3. Calcite : Not scratched by the thumbnail, but easily cut with a knife. 4. Fluorite : Can be cut with a knife, but less easily than calcite. 5. Apatite : Can be cut with a knife, but only with difficulty. 6. Orthoclase feldspar : Can be cut with a knife only with great difficulty and on thin edges. 7. Quartz : Cannot be cut with a knife ; scratches glass. 8. Topaz : Will scratch quartz. 9.

Corundum : Will scratch topaz. 10. Diamond: Will scratch corundum. QUARTZ 13 quartz porphyry, liparite, gneiss, mica schist, quartzite, and sandstones. In the granites, gneisses, and schists it occurs in the form of irregular granules destitute of crystal outlines. In the quartz porphyries and liparites it is found as a porphyritic constituent, usually with well-defined crystal outlines, which may however have become more or less obliterated through the corrosive action of a molten magma. (See Fig. 3, PI. 5.) In the secondary rocks, quartzite and sandstone, the quartz occurs as more or less rounded or irregularly angular grains without crystal outlines, except it may be through a secondary deposition of silica, as explained on p. 158. Quartz is the hardest and most indestructible of the common constitu ents, and hence when rocks containing it decompose and their debris becomes exposed to combined chemical and mechanical agencies, it remains unaltered to the very last, forming the chief constituent of beds of sand and gravel, which in turn may become transformed into sandstones, quartzites, or con glomerates.

Quartz is usually easily recognized, either under the micro scope or by the unaided eye, by its clear, colorless appear ance, irregular, glass-like fracture, — having no true cleavage, — hardness, and insolubility in any acids but hydrofluoric. Under the microscope it appears in clear, pellucid grains, often highly charged with minute cavities filled with liquid and gaseous carbonic acid, the latter like the bubble in a spirit level, dancing about from side to side of its minute chamber as though endowed with life. In other cases the cavity may be filled with a saline solution from which has separated out a minute cube of common salt. As a secondary constituent quartz occurs, filling veins and cracks in other rocks, and in the impure crypto-crystalline and amorphous forms known as chalcedony, chert, flint, opal, hyalite, and agate is found as an infiltration product in the cavities of many trappean rocks, in lenticular and oval concretionary masses in limestones, and replacing the organic matter of wood and other organisms.

The name tridymite is given to a quartz occurring in minute, usually microscopic, tablets in cavities in volcanic rocks, particularly the more acid varieties. (See further on p. 71.) The Feldspars. — Hardness, 5 to 7; specific gravity, 2.5 to 2.8. The feldspars are essentially anhydrous silicates of aluminum, with varying amounts of lime, potash, or soda, and rarely barium. They have in common the characteristics of two easy cleavages inclined to one another at an angle of 90°, or nearly 90°; close relationship in optical properties; similarity in colors, which vary from clear and transparent through white, yellowish pink, and red; more rarely greenish, and often opaque through impurities or decomposition; and lastly, a constant intergradation in composition, as already noted on p. 9. Nine varieties of feldspar are commonly recognized, which on crystallographic grounds are divided into two groups: the first, crystallizing in the monoclinic system, including ortho clase and hyalophane; and the second, crystallizing in the triclinic system, including microcline, anorthoclase, and the albite-anorthite series albite, oligoclase, andesine, labradorite, and anorthite.

The Monoclinic Feldspars : Orthoclase (Sanidin), Potash Feld spars. — Composition: K2Al2Si6016 = silicia, 64.7%; alumina, 18.4%; potash, 16.9%. This is one of the commonest and most abundant of feldspars, and forms an essential constituent of the acid rocks, such as granite, gneiss, syenite, and the orthoclase and quartzose porphyries ; more rarely it occurs as an accessory in the more basic erup tives. Under the name sanidin is included the clear glassy variety of orthoclase occurring in tertiary and modern lavas, such as trachyte, phonolite, and the liparites. Among the older rocks orthoclase not infrequently occurs in very coarse pegmatitic crystallizations with quartz and mica, and is quarried for utilization in pottery manufacture. As a rock constituent the potash feldspars are of primary importance, imparting by their preponderance, not merely color and important structural features, but on their decomposition yielding up the alkali potash, valuable for plant food, and the mineral kaolin so essential for porcelain ware, or in its impure state, as clay for pottery and brick making.