T ROCK FORMATIONS FIG. I FIG. 2 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. Micro-photograph of thin section of augite-syenite from Laurvik, Norway, between crossed nichols. The photograph shows only plain non-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, FIG. 5 Micro-photograph of thin section of graphic granite from New Bedford, FIG. 7 FIG. 8 Micro-photograph of thin section of tinguaite porphyry from the indistinct zonal structure in the same. FIG. 9 Micro-photograph of thin section of olivine norite, from Risor, Norway, showing long lath-shaped crystals of plagioclase (Pl), augite with characteristic nearly rectangular cleavage (A), olivine altered on the margins to serpentine (O1), and an opaque ore (M).. Micro-photograph of thin section of diabase between crossed nichols, showing ophitic structure. It contains plagioclase (P1), augite (A), and magnetite (M). FIG. 10 FIG IT Micro-photograph of thin section of Essexite from Gran, Norway, showing hypidiomorphic augite crystals (A), and opaque grains of magnetite (M) in a fine ground mass of plagioclase. It illustrates hypidiomorphic-porphyritic texture. FIG. 12 Porphyritic basalt (meiaphyre) with phenocrysts of plagioclase (PI) in a dense micro-crystalline ground mass consisting of augite, magnetite and a second generation of plagioclase crystals. 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 (SiO.) 3. 603 Corundum Spinel Almandite Grossular Staurolite Chiastolite Kyanite Garnet Fibrolite Cordierite Sno1 CaTiSiO Ca(CaF) (PO1)a Fe Al2(SiO) CasAl2(SiO4) Fe(AIO),(AlOH) (SiO4). Al(AIO) SiO (AIO)2SiO Al2SiO (Mg,Fe) AlSis018 Tourmaline Topaz Fluorite Zircon Allanite Zoisite ZrSiO, (CaFe)(AlCaFe) 2A1OH (SiO1): Ča2Al2(AlOH)(SiO4): 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. |