92 PECULIAR STRUCTURE OF CLARK'S CLAY-BEDS. 



It is evident that this clay was deposited under very 

 different circumstances from those under which were 

 formed the principal horizontal deposits along the river, 

 like the typical one described at the Dutchess and Col- 

 umbia Junction. At Clark's Dock there appears to have 

 been at first a deposit at least one hundred feet deep of 

 very fine sand stratified by the water. Subsequently, 

 by some change in the currents, whirlpools seem to have 

 been formed here, which, at first, were sufficiently violent 

 to excavate the conical basins in the sand ; then, as these 

 gyratory currents for some reason became far gentler in 

 their How, clay was deposited in layers on the sloping- 

 walls of these sand-basins ; as the currents became more 

 and more gentle the later layers became less concave, 

 and, at last, nearly horizontal. 



But the most peculiar feature of the locality remains 

 to be described. Outside of both the inverted cone of 

 clay and a certain thickness of the containing shell of 

 sand, at the lower levels, at least, there was a distinct 

 and remarkable vertical wall (w., figs. 1, 2 and 3). It 

 was composed chiefly of very fine sand consolidated to a 

 rock by carbonate of lime, which is evident from its 

 effervescence with acids. This wall is sometimes single, 

 but sometimes double. This vertical wall appears to 

 enclose the clay-pit on its eastern or river-front, and to 

 pass closely around it on its southern side. It did not 

 appear, however, to follow the inward curve of the clay- 

 bed on its northern side, but passed beyond its front to 

 the northward in such a direction, N. 70° E., as to form 

 a continuous front wall in a nearly straight line for all 

 three of the clay-beds on the east or river side. At the 

 time of my visits, the greater part of the northern exten- 

 sion of this wall, beyond the most southern bed, had 

 been either broken away or deeply covered up by the 

 working of the clay-beds. But Mr. Clark described to 

 me its original position, as above stated, and by careful 



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