854 A TREATISE ON METAMORPHISM. 



deposits of streams. These stream deposits are especially well illustrated 

 in semiarid regions. For instance, at the present time large masses of 

 psephites are found at the mouths of canyons adjacent to the mountains of 

 the Great Basin, southern California, etc.; and Davis,' Emmons, and Cross 

 regard it as probable that other extensive psephite formations of the 

 Cordilleras are fluviate. 



The material composing the psephites is not at all, or but roughly, 

 assorted. Consequently, with the coarse material, there is a considerable 

 amount of rather fine material to which the term psammite is applicable, and 

 of still finer clayey material belonging to the pelite order. Commonly, 

 however, the major part of the pelitic material is abstracted from the 

 psephitic material by the water. This takes place to a greater degree in 

 those deposits laid down under water than in those laid down under air. 



The material of the psephites is but poorly sorted mineralogically. 

 This is a necessary consequence of the large size of the frag-ments. The 

 majority of the larger fragments are composed of two or more minerals. 

 If the rock against which the waves are beating is granite, the pebbles 

 contain all the minerals of the granite. The same statement is true in 

 reference to all other classes of igneous and sedimentary rocks which are 

 not readily broken into fine debris. But in some instances the majority of 

 the pebbles may be composed of a single mineral. The most frequent 

 illustration of this is furnished by quartz. Finally, any of the original 

 igneous or sedimentary rocks may be in any stage of alteration, and such 

 complex mineral material constitute the pebbles, provided the alteration-is 

 not of such kind that the material breaks down into its individual mineral 

 particles. 



It follows from the foregoing that the psephites have the greatest pos- 

 sible variation in mineral and chemical composition. An organic or chem- 

 ical precipitate or an indurated clastic rock may be a chief constituent, 

 quartz may be a chief constituent, any igneous rock may be a chief con- 

 stituent, any metamorphic rock may be a chief constituent, or any of these 

 various materials may be united in any proportion. 



As rock masses, individual deposits of psephites are very common. 

 Ordinarily these masses are not extensive in area, although sometimes they 



Davis, W. M. (with discussion by S. F. Emmons and Whitman Cross), Continental deposits of 

 the Rocky Mountain region: Bull. Geol. Soc. America, vol. 11, 1900, pp. 596-604. 



