Geology of the Castle mountain mining district, Montana - Softcover

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. 1896 Excerpt: ... VI. MICROSCOPICAL PETROGRAPHY. With but very few exceptions, the igneous rocks of Castle Mountain present only well-known and well characterized types. They are confined mainly to varieties of the granitic group, and from a structural point of view they offer all the different forms of crystallization that a highly siliceous magma can assume under the most varied physical conditions; they range from a medium-grained granite on the one hand to glassy pitchstone and tuffs on the other. In the idea of volume conveyed by the superficial areas of the rock masses these acid forms far outrank all of the other varieties in the district combined. Associated with them, however, as has been pointed out in the preceding chapter, are basic rocks in much smaller amount, while rocks of intermediate character play but a subordinate role. The basic rocks are basalt and a few lamprophyric dikes, while those of intermediate composition are confined to a diorite approaching syenite and to porphyries at times inclining to porphyrites. The petrographic study of the rocks of the district is therefore chiefly that of granite, granite-porphyry, quartz-porphyry, rhyolite, and rhyolitic pitchstones and tuffs. CLASSIFICATION. In the present unfortunate state of confusion regarding the classification of igneous rocks and the use of petrographic terminology, it is generally necessary for a writer to define exactly the sense in which his terms are used. It will probably be a matter of great difficulty to obtain in the future any international concordance on this subject, but it would seem as if the times were ripe--overripe, in fact--for entirely dropping at least distinctions in classification based on the geologic age of igneous rocks. A method which is of very little practical use an...