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 (Xos. 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). 
Eocksof this class are usually 
distinctly bedded or strati- 
fied. Met amorphic rocks 
occupy in a measure an in- 
termediate position between 
those of igneous and sedi- 
mentary origin, and are 
either foliated or schistose 
(Xos. 21 to 28) through the 
parallel arrangement of their 
mineral constituents (PI. YT) , 
or massive and nonfoliated 
(Xos. 29 to 35) like marble 
and quartzite (PL 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 hi 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. 
