The Educational Series of Rock Specimens, Collected and Distributed by the United States Geological Survey
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1898 edition. Excerpt: ...In the hand specimen it may be observed that the larger streaks of black glass are distinctly perlitic, and sprinkled with phenocrysts of feldspar. The gray portion is somewhat mottled, and envelops occasionally small fragments of andesite picked up at the time the lava was teflused. Under the microscope, as seen in PI. XXX, the rock becomes plainly porphyritic, with angular fragments of feldspar, hypersthene, hornblende, and quartz embedded in a light-colored groundmass of glass, in which the fluidal structure is well displayed. The feldspar occurs in irregular tabular crystals and fragments of crystals, which generally show the twinning lamellae and angle of symmetric extinction belonging to andesine and labradorite. Polysynthetic twinning occurs according to both albite and pericline laws, but the former is much more frequent than the latter, and they are generally combined. The angle of extinction of cleavage plates on M and P ranges from zero to 20, and lies chiefly about 13, indicating labradorite. There is a small amount of sanidine present, indicated in part by the absence of twinning lamellae, but chiefly by the low specific gravity, 3.f-6, and the presence of considerable potassium, as shown by the chemical analysis of the rock. Only a portion of the potassium is to be relegated to sanidine, for the greater portion, as has been shown by chemical tests, is within the glassy groundmass. The feldspar contains numerous liquid and glass inclusions, besides a smaller amount of magnetite, quartz, pyroxene, and hornblende, which crystallized at an earlier stage than the feldspar in the solidification of the rock. A striking feature of the feldspar is the rarity of perfect crystals. In form it is irregular and fragmental, affording evidence...
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