730 GEOLOGY 



crystalline rocks, and the consequent accumulation of a heavy 

 mantle of residuary, insoluble earth. The warping which inaugu- 

 rated the Comanchean period seems to have involved a rise of the 

 Appalachian tract, and a consequent rejuvenation of the drainage 

 from it, while the coastward tract was left relatively flat, or so 

 warped as to become a zone of lodgment for the sediments brought 

 down by the quickened drainage from the west. Lakes, marshes, 

 etc., were probably features of the lodgment area. These conditions 

 are in harmony with the constitution of the deposits, which consist 

 of gravel (or conglomerate), sand (or sandstone), and clay, largely 

 uncemented. 



The gravel at an}^ point is made up principally of materials 

 derived from formations adjacent on the west, and subordinately 

 from subjacent formations. It is often arkose in the immediate 

 vicinity of the feldspar-bearing crystalline rocks, but elsewhere it 

 is composed chiefly of the resistant products of mature weathering. 



The sands are sometimes fine and the grains well rounded, as 

 if transported far by moving water, and sometimes coarse and 

 angular, as if they had been subjected to but little wear. Locally 

 they contain feldspar grains, or bits of kaolin which have resulted 

 from their decay. The presence of the feldspar in the sand, like 

 the presence of pieces of schist in the gravel, shows that erosion 

 sometimes exceeded rock decay. This betokens high land to the 

 west whence the sediments were derived, and is one of the reasons 

 for the belief that the region west of the site of deposition was t ilted 

 upward at this time. 



Beds of clay of such purity and magnitude arc found in the 

 Potomac series, that they have been extensively utilized, especially 

 in New Jersey, 1 for the manufacture of clay wares. The clay often 

 shows little stratification, and is notable for its bright and varie- 

 gated colors, black, white, yellow, purple, and red being not un- 

 common. White is to be looked upon as the normal color; the 

 others are the result of various impurities, the black being due to 

 organic matter. 



The clay, sand, and gravel are irregularly disposed, dnul>tle<- 



, Report on Clays, Geol. Surv. of Now .J.-rsry, Isivs. : m.l Kimum-I, 

 1904. 



