ROCK FORMATIONS FIG. FIG. 1. Micro-photograph of thin section of granite, from Lokken, Norway, between crossed nichols, showing patchy appearance of soda-orthoclase (O), irregular plates of quartz (Q), and illustrating granitoid (xenomorphic or allotrimorphic) texture. The orthoclase shows twinning. 2. Micro photograph of thin section of augite syenite from Laurvik, Norway, between crossed nichols. The photograph shows orly plain nor-striated orthoclase and illustrates granitoid allotrimorphic or xenomorphic texture. The lines due to cleavage. FIG. 3. Micro-photograph of thin section of norite, from Hitteroe, Norway, between crossed nichols, showing banded plagiocase Labradorite (P), an opaque ore, ilmenite (I), and a monoclinic pyroxene diallage (D). The section also illustrates granitoid (xenomorphic or allotrimorphic) texture. A fringe of biotite surrounds part of the ore. 4. Micro photograph of thin section of Perthite, between crossed nichols, showing intergrowth of albite (Ab) and microcline Mi The albite shows banding and the microline cross-hatching. 5. Micro-photograph of thin section of graphic granite from New Bedford, Mass. between crossed nichols, showing parallel intergrowth of quartz (Q) and micro cline (M). The latter shows cross-hatching. FIG. 6. Micro-photograph of thin section of porphyritic perlite showing sanidin crys tals (S) in glassy ground mass. Dark wavy bands indicate flow in the mass before cooling (flow structure). This section of porphyritic trachite showing sanidir (S) and biotite (B) in glassy ground mass. The crystals have been co-eroded or re dissolved by the magma as is shown by their even carved boundaries. FIG. 8. Micro-photograph of thin section of tinguaite porphyry from the Odenwald showing diomorphic crystals of nepheline (N), a'so an indistinct zonal strutare in the same. FIG. 9. Micro-photograph of thin section of olivine norite, from Risor, Norway, showing long lath-shaped crystals of plagioclase (Ph, augite with characteristic nearly rectangular cleavage (A), olivine altered on the margins to serpentine (Ol), and an opaque ore (M). FIG. 10. Micro-photograph of thin section of liabase between rossed nichols, showing opatic structure It contains plagioclase (P), augite (A), and magnetite (M). FIG. II. Micro photograph of thin section of Essexite from Gran, Norway, showing hypidiomorphic angite crystals (A), and opaque grams of magnetite (M) in a fine ground mass of plagioclase. It illustrates hypidiomorphic-porphyritie texture. FIG. 12. Porphyritic basalt (meiaphyre) with phenocrysts of plagioclase (P) in a dense micro-crystalline ground mass consisting of augite, magnetite and a second generation of plagioclase crystals. ROCKS phelite, having the composition K2Na,Al, Si,O. Chemically leucite resembles 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. GROUP IV. Thus far the mineral species enumerated are mostly found as the essential constituents of that class of rocks hereafter to be described as ig neous. 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. Optically it is distinctly biaxial. 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 is very nearly uniaxial. It has the chemical composition (HK)2 (MgFe)2 Titanite (AlFe).(SiO.)3. GROUP V. The Ferromagnesian Minerals.-The minerals of this group take their name from the fact that they contain iron and magnesia as two of their prominent constituents. In addition they may contain lime and alumina in considerable amounts, also some alkali. The more important members of the group are included under the amphiboles and the pyroxenes, two species having numerous varieties, which pass into each other by isomorphous mixture. They form two parallel series which are both chemically and crystallographically analogous. Their relationship can best be shown by the following table: GROUP VII. Accessory Minerals.-Those minerals which are usually of subordinate importance, and which are therefore more in the nature of accessory than notable rock-constituents, are included here: Cassiterite Sno (Ce,La,Di) PO Apatite Ca(CaF) (PO)3 Corundum Spinel MgAl2O Almandite FesAl2(SiO4)3 Garnet Grossular CasAl(SiO4)3 Staurolite Chiastolite Kyanite Topaz Zircon Fe(AIO), (A1OH) (SiO1), (AIO) 2SiO3 AlgSiO (Mg,Fe) Al, Si,O18 (Na Mg Al)(AlFe).(BOH),SiO38 (AIF)2SiO CaF2 ZrSiO (CaFe),(AlCaFe) 2A1OH (SiO4)3 CagAl (AIOH) (SiO4)3 Some of the above minerals occasionally come to be of considerable importance, or may even constitute the bulk of the rock-mass. As a rule, however, they are of minor consideration and frequently even negligible. GROUP VIII. 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 Talc Epidote HA12SiO Hs (Mga, Fe) Al2Si3018 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 limemagnesia or non-aluminous ferromagnesian minerals, namely, tremolite, pyroxene (chiefly enstatite), phlogopite mica. Chlorite results most ROCKS 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 Professor 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, discrete 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. Rock Disintegration.- Granite is one of the most abundant and widespread of igneous rocks, and is a most important species among the numerous other primitive rocks of the lithosphere. The changes which take place in it in the process of its decay can be taken as illustrative of those which take place in the disintegration of all other primitive rocks. Granite consists of quartz, orthoclase feldspar (with some sodaorthoclase microcline or plagioclase), and light or dark mica, or both, or perhaps hornblende in place of the micas. On decomposition under the influence of atmospheric agencies, it falls to a more or less rusty, clayey mass of sandy or gravelly material, the sandy or gravelly part consisting of angular fragments of the original quartz (which is practically unaffected by atmospheric agencies), and fragments of still undecomposed feldspar. The clayey portion of the alteration products results from the decomposition of the feldspar and consists largely of kaolin, which in a pure condition is a white powdery or plastic material, according to whether it is dry or wet. It is usually stained rusty brown by iron oxide, which results from the decomposition of any ferromagnesian constituent contained in the rock, or from small particles of some one of the ores which are quite certain to have been present in small amounts. Certain constituents of the original minerals are carried off in solution: the alkalies and a part of the silica the feldspars; also a portion of the iron of the ferromagnesian minerals, together with some of the magnesia and much of the lime that may have been present as a minor constituent. This mass of loose, more or less rusty, incoherent material remaining behind is termed residual granite. Rocks of whatever nature are in like manner subject to decomposition by atmospheric agencies, and their residual materials everywhere cover the greater portion o: the surface of the underlying rocks, and constitute what is termed mantle-rock or detritus. A part of this mantle-rock consists not of residual material in the strict sense, but is made up of angular fragments of various sizes, which have been broken from the rock-masses by the action of frost. At the foot of nearly every steep cliff is to be found an accumulation of angular rockfragments called talus, rock-slide, or breccia. Removal of Rock-waste and Its Deposition in the Form of Sediments. Most of the various materials of the mantle-rock, whether residual or fragmental, find their way sooner or later, chiefly by the action of rain or frost, to the neighboring streams, and are borne by them to the rivers, which in turn transport them, after numerous halting-periods, to the sea or to smaller bodies of salt or fresh water. In this process of transportation the angular rock-fragments are reduced by attrition to rounded pebbles. The angular grains of quartz also become rounded and water-worn, while much of the material becomes reduced to an impalpable mud. These more or less finely comminuted and abraded materials are distributed along the shores of lake or sea. Gradually the finer, more easily suspended and transportable materials are carried out into deep water, the finest and most impalpable muds being transported farthest from the shore; so that, broadly speaking, the washings from the land surface become distributed over sea and lake bottom in order of fineness, beginning with the coarsest gravelly materials at the shore-line, and growing successively finer toward the deep water, where finally only the impalpable silts and muds are deposited. Stratification. In addition to this more or less gradual horizontal change from coarse to fine materials brought about by the transporting power of water, there is always to be observed a much more sudden and abrupt change vertically, by which materials of various sorts and degrees of fineness are arranged by the sorting power of the same medium (water) into horizontal beds of varying thickness, which are separated from each other by sharply defined planes of demar. |