THE COMANCHEAN PERIOD. 113 



the sand-beds are sometimes rather lawless in their disposition. Locally 

 the sand contains feldspar grains, or bits of kaolin which have resulted 

 from their decay. The presence of the feldspar (or kaolin) 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 tilted upward 

 at this time. 



Much of the feldspar of the crystalline rocks was already decom- 

 posed at the time of . the Potomac sedimentation, and the resulting 

 clay was often separated, in deposition, from the coarser grains of 

 quartz. This separation was the work of the waters which trans- 

 ported the detritus, and while it was effected by physical means, and 

 for physical reasons, it resulted in the separation of materials which 

 were chemically unlike. The separation was by no means always 

 complete; but it went sufficiently far to give rise to beds of clay of 

 such purity and magnitude that they have been extensively utilized 

 (especially in New Jersey x ) for the manufacture of clay wares. The 

 beds of clay, like those of gravel and sand, are sometimes in the form 

 of huge lenses. The clay often shows little trace of stratification, 

 and is notable for its bright and variegated colors, black, white, yel- 

 low, purple, and red being not uncommon. White is to be looked upoo 

 as the normal color; the others are the result of various impurities, 

 the blackness being due to organic matter. 



The irregular disposition of the clay, sand, and gravel is doubt- 

 less the result of the physical conditions where the sedimentation 

 took place. On an exposed coast, the waves and littoral currents 

 tend to spread the coarse sediment along the shore, while the finer 

 sediments are carried farther out. Where the Potomac sediments 

 were deposited, such processes appear not to have been effective, and 

 the sediments vary notably from point to point. Their disposition 

 is often such as to suggest that they were deposited along the lower 

 courses of rivers or at their debouchures, where shore-waters had 

 little effect upon them. On the other hand, the perfect separation 

 of the sand from the clay in many places, points to the existence of 



1 Cook, Geol. Surv. of New Jersey, Report on Clays (1870), and Kiimmel, Ries, and 

 Knapp, 1904. 



