ROCKS orthoclase feldspar. In addition to being chemically like orthoclase, leucite, like its counterpart, alters to kaolin. But more interesting still is the fact that on decomposition it has been known to furnish orthoclase or orthoclase and muscovite. Nephelite, as its chemical composition indicates, is analogous to the plagioclase feldspars. It is hexagonal in character. The decomposition of nephelite, like that of the plagioclases, usually results in the formation of some one of the zeolites, or more rarely it forms kaolin. The feldsparthoids are lower in silica than their corresponding feldspars, and it is believed that, where they are present, true feldspars would have formed, except for this deficiency in silica. They hold an important place in the classification of rocks. GROUP IV. Mica.- Two species here are of widespread occurrence; one is muscovite, the white, silvery, potash variety, having a chemical composition corresponding to H,KAl(SiO.).. Chemically it is closely allied to orthoclase, and frequently results as an alteration product of that mineral. The second species, biotite, is dark-colored, owing to comparatively high percentages of iron. It also contains varying amounts of magnesia, and might thus be properly classed with the following group of minerals. It has the chemical composition (HK),(MgFe), (AlFe) 2 (SiO4)3. 603 Secondary Minerals.- Minerals of this class have originated as the result of the decomposition or alteration of some previously existing primary or original mineral. The most noteworthy are: Kaolin Serpentine Talc Chlorite Epidote H.AlSi2O, CazFe2(FeOH) (SiO4): Kaolin results from the decomposition of orthoclase and other feldspars by the loss of some of the silica and alkalies, and by the addition of water (hydration). Serpentine results from the alteration of olivine and the nonaluminous hornblendes and pyroxenes by the loss of some of the magnesia and by an addition of water. Talc is formed by the hydration and partial decomposition of several of the lime-magnesia or non-aluminous ferromagne Origin of Rocks.- Two alternative hypotheses are at present recognized as possible explanations of the origin of the earth: (1) the old so-called nebular hypothesis, propounded by Kant and Swedenborg and later elaborated by Laplace and others; (2) the newly proposed accretion theory or planetesimal hypothesis propounded by Prof. Thomas C. Chamberlin of the University of Chicago. The first of these supposes that the earth was originally in a gaseous condition, from which, under its own gravity and by a radiation of its heat, it passed into a fluid state and thence to a solid form. The "original crust" of the earth (lithosphere) formed at the surface as the result of a cooling of the molten materials of the globe. Furthermore, the earth may have solidified from the centre outward as the result of pressure. Others suppose that the original crust was not only added to from below by the crystallization of molten material, but also increased in thickness from above by chemical precipitations from the intensely heated hydrosphere. The accretion theory, on the other hand, supposes that the earth as a whole never was in a gaseous or even fluid condition, but was built up by the infalling of cold, solid particles of matter called planetesimals; that the present internal heat is the result of pressure due to gravity. Adherents of both these hypotheses agree, however, that the oldest known rocks, the original or primitive rocks from which all others have been derived, are of igneous origin, that is, were once in a molten condition, from which they cooled to the solid state, and in so doing formed more or less thoroughly crystalline aggregates of different kinds of minerals. In discussing the origin and descent of rocks we must, therefore, start with the more common igneous varieties, and show how they have furnished materials for the others. Rocks may be classified as (1) igneous, (2) sedimentary and (3) metamorphic. IGNEOUS ROCKS. (Definitions depending upon external form and mode of occurrence.) Igneous rocks comprise all those portions of the lithosphere which are or were in an intensely heated and more or less fluid condition and which have subsequently solidified by cooling into massive, more or less crystalline bodies of varying sizes and shapes. They occur as intrusions into or replacements of some previously existing rocks of the lithosphere, either sedimentary, meta morphic or igneous. Where a huge mass of molten material slowly melts its way from some deep-seated portion of the earth up into the overlying rocks by dissolving them and incorporating them in its own mass, and subsequently cools there, slowly, without ever reaching the surface, forming a great irregular body of coarsely crystalline matter frequently scores of miles in extent, we have what is termed a batholith. In its molten condition it would be termed a magma and the reservoir in which it was contained would be called a magma basin. It might have extending from it (usually upward) irregular, more or less elongated, armlike processes called apophyses. Should cracks or fissures form, they would be instantly filled by the inrush of fluid substance, which would solidify with comparative rapidity to form dikes. In stratified rocks dikes usually cut at some angle across the strata. Where the molten material intrudes itself between the strata parallel to the bedding planes in broad sheets, it is called an intrusion-sheet or sill. Where the overlying strata become lifted and arched upward into a huge dome without breaking above and letting the fluid rock escape, the solidified lenticular mass constitutes a laccolith or laccolite. Where the molten rock actually escapes to the surface by means of a fissure or other conduit, and flows out over it in a broad sheet, it is termed a flow, lava-flow, or extrusion-sheet. The interval of time elapsing between the outbreak of the magma from its reservoir and its subsequent cooling to the solid state is termed the effusive period. If the conduit is rudely cylindrical the lava accumulates about the vent, forming a lava or volcanic cone. Subsequent erosion may entirely remove the cone and expose the cold lava in the conduit, to which the term volcanic neck is applied. In fact it is only through the exposure of these deep-seated masses of cooled igneous material by erosion that we are enabled to study them. Over the more ancient land areas many miles of rock, vertically measured, have been removed, laying bare the underlying rocks to a corresponding depth. - The Modern Conception of Rock Magmas. The popularly accepted idea of lava is that it is rock which has been fused by great heat. In fact the older conception of scientific men was that magmas consisted of fused rockmasses. Recent opinion, however, based on the revelations of the microscope and experiments in the synthesis of minerals and rocks, tends to regard them as solutions of one mineral substance in another or of several mineral substances mutually in each other. One of the chief reasons for thinking them such can be stated as follows: If magmas are fusions then the individual mineral constituents potentially present in them should crystallize out according to their fusibilities, beginning with the least fusible, but they do not. On the other hand, as the magma slowly cools, the first individual mineral species makes its appearance when the point of saturation for that particular species is reached, and the others follow in the order of their solubilities. For example, in granite (which consists of quartz, feldspar and mica or hornblende, with some accessory constituent, such as apatite or zircon, and, it may be, also a small amount of one or more of the ores), ROCKS the order of crystallization is: (1) the ores with apatite and zircon; (2) mica or hornblende; (3) feldspar; (4) quartz. Quartz is the least fusible, and should appear first under the fusion theory. As a matter of fact it appears last. In short, all of the materials appear in the reverse order of what we would expect if they are true fusions. But they do appear in the order of their solubilities. Pressure, however, exerts an important influence upon the solvency of a substance, and as this is variable the order of appearance is not absolutely fixed. Moreover, heat, the most important factor involved, is very variable, and still farther modifies the problem. The Splitting or Cleavage of Magmas (Spaltung, Magmatic Segregation, Magmatic Differentiation). When a magma cools, the first minerals to appear are the ores (with apatite, titanite, zircon, etc.); following these, the basic ferromagnesian constituents; then the more basic plagioclase feldspars, followed or overlapped by the orthoclase varieties; and last of all, quartz, if more than enough exists for the formation of the silicates. Obeying the principle of diffusion, the first minerals to form the more basic ones- - tend to accumulate on the cooling walls, that is, at the periphery of the magma-basin; and the other minerals arrange themselves rudely in concentric zones, each zone toward centre being successively more acid, until at the centre the magma becomes comparatively acid and may, after solidification, consist largely of such minerals as orthoclase and quartz. This arrangement of the minerals by diffusion before crystallization results in the rude separation of an originally uniform magma into several magmas of different chemical compositions, which, on cooling, furnish rocks of different mineralogical compositions. The process is termed the cleavage of magmas. Definitions depending upon internal characteristics will next be considered. Texture. The fluid magmas of igneous rocks may be compared to molten glass. If the cooling period be long the individual minerals form comparatively large crystals. If it be short the crystals are correspondingly small. If it chills so suddenly that the molecules of the different mineral compounds do not have time to unite to form crystals, but are caught just as they existed originally in the fluid magma, the result is a volcanic glass. Rocks in which the crystals are five millimeters or more in diameter may be called coarse-grained; rocks in which the crystals range in size from one to five millimeters, medium-grained; and those in which they are one millimeter or less in diameter, finegrained. Rocks in which all of the original nagma has individualized or crystallized, to form minerals of some sort, and in which there is no unindividualized material remaining behind in the form of glass, are called holocrystalline. All rocks in which the crystals are large enough to be seen with the unaided eye are termed phanerocrystalline, or phaneric. Those rocks in which the crystals are too small to be distinguished megascopically are called aphanitic. Many aphanitic rocks, however, under the microscope are seen to be holocrystalline and to consist of small crystals of minerals which can be specifically identified with the aid of that instrument. For these rocks the 605 term microcrystalline has been proposed. But the crystals, though recognizable, may be too small to be specifically identified even with the microscope. Such rocks are called microcryptocrystalline. Volcanic glasses show only embryonic crystals imbedded in textureless glass. Such rocks are said to be vitreous or glassy. If during the entire period of solidification the conditions of cooling remained the same and the entire process of cooling of a deepseated magma was slowly and quietly accomplished in the magma-basin, unattended by any effusive period, the rock would be coarse (to fine), evenly-granular. In rocks cooling under these circumstances the growing crystals have usually interfered with each other in such a way as mutually to destroy their crystal boundaries, forming irregular interlocking grains, with no one diameter much greater than the others. In other words, the grains are irregularly rounded. A texture of this sort is characteristic of the granites, and is called the granitic or granitoid texture. The term allotrimorphic, referring to the fact that the crystals do not possess their own boundaries, is applied to the same texture, and a still newer term, xenomorphic, has recently been proposed. Where the majority of the crystals do retain their crystal boundaries, the resulting texture is called idiomorphic or automorphic. If the crystal boundaries are only faintly or imperfectly discernible, the structure is said to be hypidiomorphic. The period of solidification within the earth may be interrupted by an effusive period; in which case those crystals which had begun to form, and may have reached considerable size are carried by the eruption into other surroundings, where the cooling process may be much accelerated. A second generation of smaller crystals would then form about the large well-formed ones, imbedding them in a finegrained ground-mass, producing what is termed porphyritic texture. Two or more generations of crystals may thus be recognized. The welldefined crystals of the first generation are called phenocrysts. The term felsite is applied to the fine-grained ground-mass of the acid rocks. The crystals of some one mineral species in a rock may have one diameter much larger than the others. The crystals then appear distinctly lath-shaped in thin section under the microscope. This form of crystallization is especially common with the plagioclase feldspars. These laths of plagioclase are enclosed by crystals of another species. This texture is common, and in the scheme of classification of igneous rocks at present accepted is called into rather prominent requisition. It is known as the ophitic texture. Flow-structure is exhibited by the parallel arrangement or orientation of minerals in lines which indicate the direction in which the fluid rock had been moving before solidification took place, or while it was in a viscous state. Classification of Igneous Rocks.-The classification of igneous rocks as at present widely accepted is based upon three things: (1) chemical composition; (2)_mineralogical composition; (3) texture. (The proposed Quantitative Classification of Igneous Rocks, published jointly by Cross, Iddings, Pirsson and 1 |