British Petrography: With Special Reference to the Igneous RocksDulau & Company, 1888 - 469 Seiten |
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acid actinolitic aggregates albite ALLPORT alteration analyses andesites apatite appears augite axis Band basalts basic biotite brown character chemical chlorite cleavage colour colourless composed composition consolidation containing Cornwall crossed nicols crystalline described diabase diallage diameters diorite district dolerites double-refraction dykes enstatite epidote extinction felsites felspar ferro-magnesian foliated frequently gabbro glass granite granophyres granular green ground-mass H₂O hornblende idiomorphic igneous rocks inclusions interstitial matter intrusive iron-ore irregular grains labradorite lath-shaped sections latter lavas less magma magnetite mass metamorphism mica microlites microscope minerals nepheline olivine ophitic optic original orthoclase parallel petrographical picrites plagioclase planes plates pleochroism plutonic porphyritic porphyritic constituents porphyritic crystals portions present prisms produced pseudomorphs pyroxene quartz referred refraction represented rhombic right angles rocks occur ROSENBUSCH schistosity schists secondary seen serpentine silica sometimes specific gravity specimens spherulites structure substance term texture thin section tourmaline trachytic twinning varieties veins volcanic zone
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Seite 214 - ... a little space might doubtfully be entitled to the appellation of greenstone, and it is, therefore, more than improbable that a deep-seated mass, which, when cooled, crystallized into greenstone, should have overflowed in lava beds of purely felspathic quality. Their ingredients are different, and in no single case in Merionethshire is there any line of greenstone that can be proved to have flowed across the surface. There may be difficult and doubtful cases, and there are many in which a cursory...
Seite 120 - ... that is, from his point of view; from mine it is perfectly natural. He goes on to say : " Whatever the cause of this apparent passage may have been, it is very readily...
Seite 437 - A term used in describing the character of the reflections obtained from the fractured surfaces of minerals and rocks. There are several kinds of lustre — metallic, adamantine, greasy, vitreous, resinous, pearly, silky, etc.
Seite 374 - They, too, vary in ¡size from that of a pin's head to that of a pea, and project but little over the surface of the membrane, and are therefore sometimes scarcely visible.
Seite 433 - By hardness is understood the resistance offered by a body to the separation of its particles. The hardness of a mineral is measured by the force required to scratch it with a steel point or pointed fragments of certain standard minerals.
Seite 389 - On approaching the altered area, the slate first becomes faintly spotty, the spots being of a somewhat oblong or oval form, and a few crystals of chiastolite appear. Then these crystals become more numerous, so as to entitle the rock to the name of Chiastolite Slate. This passes into a harder, more thickly bedded, foliated and massive rock, Spotted (or Andalusite) Schist ; and this again into Mica Schist of a generally grey or brown colour, and occurring immediately around the granite.
Seite 178 - Although it is convenient to use the expressions flaser-gabbro and gabbro-sohist, it must not be supposed that there is any hard and fast line between the rocks designated by these terms. They shade into each other by the most insensible gradations. One very interesting structural variety which is, in some respects, intermediate between flaser-gabbro and gabbro-schist, may be termed augen-gabbro. In this variety the streaks which define the schistosity sweep round " eyes " of diallage, which thus...
Seite 121 - ... trace has yet been discovered of either serpentine or diallage rock, though the latter especially is of great hardness, and therefore quite as likely as the hornblende rock to resist comminution into fine sand, and the two former are nearer to the conglomerate than the latter.
Seite 386 - in numerous localities we find the coarser slates converted into rocks resembling mica-slate and gneiss, a fact particularly well exhibited in the neighbourhood of Meavy, on the south-east of Tavistock,' and ' near Camelford we observed a fine arenaceous and micaceous grauwacke turned into a rock resembling mica-slate near the granite.