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. 1892 edition. Excerpt: ...was contemporaneous in part. As there are two or more generations of plagioclase and pyroxene, it is natural that the hornblende appears to be contemporaneous with the earlier feldspars and pyroxenes, and older than the later generations. The feldspars are all plagioclase, but appear to belong to different ...
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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. 1892 edition. Excerpt: ...was contemporaneous in part. As there are two or more generations of plagioclase and pyroxene, it is natural that the hornblende appears to be contemporaneous with the earlier feldspars and pyroxenes, and older than the later generations. The feldspars are all plagioclase, but appear to belong to different species. They are in rather small crystals in most of the rock sections. The larger ones are generally labradorite, and many of the small ones are the same, but in a number of the sections the extinction angles indicate andesine or ohgoclase. They are mostly rectangular with perfect crystallographic outline, some are tabular and polygonal, and in this position they exhibit the most striking zonal structure, which is almost universally present. The twinning is that characteristic of andesitic plagioclases. Glass inclusions are of frequent occurrence. In some cases the feldspar contains a great amount of glass which almost equals the bulk of the feldspar substance. Occasionally the feldspar has an irregular form and an indented outline, made by the projection of crystal points, the margin of the individual having a different optical orientation from the central portion, and appearing to be formed of more alkaline plagioclase. These are not very common. The groundmass of these andesites is the same as that of the pyroxene andesites in some cases, and is composed of globulitic brown glass with microlites of pyroxene, feldspar and magnetite. But in most of the sections it consists of colorless glass crowded with small microlites of the same minerals. It carries microscopic crystals of these minerals which are porphyritical with respect to the groundmass when seen with a microscope, but which in turn form part of the groundmass which carries the...
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