Sentence Examples


  • The rocks of its lower half are mainly granite and gneiss; its upper half is composed of porphyritic greenstone, and a variety of minerals occur.
  • Few obsidians are entirely vitreous; usually they have small crystals of felspar, quartz, biotite or iron oxides, and when these are numerous the rock is called a porphyritic obsidian (or hyalo-liparite).
  • Porphyritic crystals often contract less than the surrounding glass, which accordingly becomes strained, and in polarized light may show a weak double refraction in a limited area surrounding the crystal.
  • Above Lima the western chain of the Andes is composed of porphyritic tuffs and massive limestones, while the longitudinal valley of the Oroya is hollowed in carbonaceous sandstones.
  • The Eastern Cordillera is composed of gneiss, mica and chlorite schist and other crystalline rocks of ancient date; the Western Cordillera, on the other hand, is formed of porphyritic eruptive rocks of Mesozoic age, together with sedimentary deposits containing Cretaceous fossils.
 

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