chemical and geological essays
by thomas sterry hunt
Added by: Jonathan W. F...
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...entirely wanting, and is never abundant in any large mass of this gneiss, although small bands of mica-schist are occasionally met with. Argillites, which from their general predominance of potash and silica are related to the first class of sediments, are, so far as known, wanting throughout the Laurentian series ; nor is any rock here met with, which can be regarded as derived from the metamorphism of sediments like the argil- lites of more modern series. Chloritic and chiastolite-schists and kyanite are, if not altogether wanting, extremely rare in the Laurentian system. The aluminous sediments of the second class are, however, represented in this system by a diabase made up of dark green pyroxene and bluish labradorite, often associated with a red alumino-ferrous garnet. This latter mineral also sometimes constitutes small beds, often with quartz, and occasionally with a little pyroxene. These basic aluminous minerals form, however, but an insignificant part of the mass of strata....
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