. Bulletin of the Department of Agriculture. Agriculture; Agriculture. BULLETIN 348, U. S. DEPARTMENT OF AGRICULTURE. rocks (Nos. 1 to 7), such as granite, diorite, gabbro, etc. (Pis. I, fig. 1; II, figs. 1, 2), having cooled slowly at great depths below the earth's surface, usually possess large and well-developed mineral components, whereas extrusive or volcanic varieties (Nos. 8 to 14), cooling more rapidly upon the earth's surface, are finer grained and frequently show so-called porphyritic structures owing to the development of larger crystals in a dense, fine-grained ground mass. (P
. Bulletin of the Department of Agriculture. Agriculture; Agriculture. BULLETIN 348, U. S. DEPARTMENT OF AGRICULTURE. rocks (Nos. 1 to 7), such as granite, diorite, gabbro, etc. (Pis. I, fig. 1; II, figs. 1, 2), having cooled slowly at great depths below the earth's surface, usually possess large and well-developed mineral components, whereas extrusive or volcanic varieties (Nos. 8 to 14), cooling more rapidly upon the earth's surface, are finer grained and frequently show so-called porphyritic structures owing to the development of larger crystals in a dense, fine-grained ground mass. (Pis. I, fig. 2; III, figs. 1, 2.) Sedimentary rocks (Nos. 15 to 20), on the other hand, are made up of mineral or shell fragments that have been transported mainly by water and deposited in sea or lake depressions in more or less parallel layers and subsequently cemented together through pressure' and the formation of sec- ondary mineral compounds. (Pis. IV, fig. 1; V, fig. 2). Rocks of this class are usually distinctly bedded or strati- fied. Metamorphic rocks occupy in a measure an in- termediate position between those of igneous and sedi- mentary origin, and are either foliated or schistose (Nos. 21 to 28) through the parallel arrangement of their mineral constituents (PL VI), or massive and nonfoliated (Nos. 29 to 35) like marble and quartzite (PI. V, fig. 1). These structures have been induced by long-continued shearing and compressive forces acting alike on igneous and sedimentary rocks, thereby changing to a greater or less degree the composition and structure of the original rock masses. MINERAL COMPOSITION OF ROCKS FOR ROAD MAKING. Although the road materials tested in this office have been separated into 35 distinct varieties, yet it will be seen that but 12 minerals enter prominently into their composition. Some idea of the general appearance of these minerals when perfectly developed may be obtained from the photographs reproduced on Plate VII, yet the fact should be
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