96 PECULIAR STRUCTURE OF CLARK'S CLAY-BEDS. 



within the strong wall was mostly undisturbed ; but 

 that without was changed to coarse sand and pebbles, 

 whose layers accommodated themselves to the wall, as 

 the facts show. Though this wall, solid as it was, could 

 not have held up the pressure of the inner sand, had 

 these changes taken "place out of water, yet while under 

 water the pressure of the water without would have 

 gone far to relieve the strain. At some points of weak- 

 ness or incompletion, however, the coarse pebbles found 

 their way inside of the wall at its lower parts. Subse- 

 quently, the wall was completed across these pebbly 

 portions. The difference in smoothness between the 

 inner and outer surfaces of the wall may perhaps be ex- 

 plained by the difference in the coarseness of the material 

 on the two sides. 



The only other possible explanation that has sug- 

 gested itself to me is that of a fault, by which the out- 

 side coarser material was forced up so as to cut the sand- 

 bank smoothly in a vertical plane. The carbonated wa- 

 ters, derived from sources stated above, falling along 

 this plane, thus determined, may have formed the wall 

 in question. 



That such a fault is possible would appear from an ob- 

 servation made by Prof. Mather, Geol. of 1st Dist., N. 

 Y., {State Rep. p. 156), where he speaks of " a fault in 

 the clay and gravel-beds near Newburgh, where the clay 

 and sand, horizontally stratified, were separated by a 

 vertical line on the surface exposed, each abutting 

 against the other with little disturbance of either, and 

 covered by beds of coarse gravel, evidently deposited 

 after the fault had been formed. This locality was 

 where there had been no slide nor derangement of the 

 strata since the deposit of the gravel-beds, and where 

 no present causes could wash the gravel." This locality 

 is "half or three-quarters of a mile below Newburgh on 



the shores of the Hudson." 



so 



