Guide to the Field Columbian Museum, with Diagrams and Descriptions - Tapa blanda

Sinopsis

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.1894 Excerpt: ... ties, etc.), down to the amorphous rocks. Thus, beginning with the granites, which have from 80 to 65 per cent. of silica, we pass among the coarse grained rocks to the diorites, which have between 65 and 55 per cent., then to the gabbros and diabases, having usually more than 45 per cent., and end with the peridotites, having below 45 per cent. A corresponding series begins with the syenites, and ends with the nepheline rocks. Cases 1, and part of 2.--First row, granite and its varieties, such as granitite, graphic granite, etc. These are rocks having quartz, alkaline feldspar, and one or more minerals of the mica, amphibole, or pyroxene groups as essential constituents. Second row, granite-porphyry, quartz-porphyry, vitrophyre, fehophyre, etc. Like the preceding in composition, but more or less porphyritically developed. Third row, rhyolite, nevadite, pumice, obsidian, etc. These are amorphous volcanic rocks, having high percentages of silica, usually more than 70 per cent. Upper Part of Cases 2 and 3.--Polished slabs, chiefly granite, with some marbles. Remainder of Case 2, and Case S.--Syenite-nephelinite series. First Group.--First row. Syenite, minette, etc. Holocrystalline rocks, having orthoclase and biotite as essential constituents. Second row. Trachytes. Tertiary eruptive rocks, characterized by the predominance of an alkaline feldspar, usually sanidine, and freedom from quartz. An iron bearing mineral is also usually present. Second Group.--First row. Nepheline or elaeolite syenites, rocks comprised of nepheline, orthoclase, and usually a pyroxenie mineral and plagioclase feldspar. Second row. Phonolites, rocks consisting of an alkaline feldspar, with minerals of the nepheline and leucite groups, and usually a monoclinic augite. Third Group.--Te...

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