CONGLOMERATES. 855 



are. Generally they are of but moderate thickness. The psephite deposits 

 made by standing bodies of water are usuall)* in narrow bands adjacent to 

 the shore, and, moreover, occur only locally along the shore. -Where, 

 however, a body of water steadily transgresses over the land, as, for instance, 

 the ocean, a continuous formation of psephite over an extensive area mav 

 be deposited. Such a formation, while continuous, is not of the same age 

 throughout. The earlier portion of a formation is usually buried under 

 other sediments by the time the later part of the formation is deposited. 

 Usually such psephites are not very thick formations — that is, hundreds of 

 meters — for, as the land subsides and the sea transgresses, the waves can 

 not transport the coarse material back to the areas where earlier psephites 

 were deposited; consequently they are buried under finer material. The 

 psephite deposits of continental glaciers may be very extensive and of 

 considerable thickness. But psephite deposits of this class found among 

 the older rocks are not very numerous. This may be partly due to their 

 destruction by later geological action. The psephite deposits of fluviate 

 origin, while usually of rather local extent, may be of considerable thick- 

 ness, often from one hundred to several hundred meters. How far deposits 

 of this origin are represented among the older formations which have been 

 buried by later deposits it is difficult to say. 



CONGLOMERATES. 



Pebble, gravel, and bowlder deposits, by consolidation, cementation, 

 and metasomatism, are transformed to conglomerates. (PI. VIII, A.) These 

 processes take place in the belt of cementation. The rocks here considered 

 are so coarse that consolidation due to pressure in -the belt of cementation 

 produces comparatively little effect, since the coarse fragments have com- 

 paratively few points of contact. The process of cementation is identical 

 with that of the psammites, described below. Since the psammites are 

 more important than the psephites, the process of cementation is more 

 fully discussed under that head. Because of the very great variety of 

 material composing' the psephites upon which the ground waters are acting, 

 there may be a great variety of minerals deposited in the interstices. But 

 the most abundant cement is usually quartz. Next in importance to quartz 

 are iron oxide and the carbonates. In some cases hornblende, feldspar, 

 etc., may be of consequence. 



