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 borne in mind that when present as rock-forming 

 aggregates these regular crystal shapes are considerably modified 

 and the minerals appear in more or less irregular forms depending 

 upon the character of the rock in which they occur. (Pis. I— III.) 



Fig. 2. — Cross-line field illustrating method of estimating 

 minerals in thin section. 



