rlOO 



NEW YORK STATE MUSEUM 



had been so thoroughly weathered that the only coarse fragments 

 available for building conglomerates were purely quartzose the 

 feldspars, ferro-magnesian minerals, etc. having been decomposed. 



The finer parts of these residual clays can not now be located; 

 but it is natural to assume that they somewhere formed shales, 

 now buried under later sediments. Some of the clay, too, may 

 have been mingled with the sand, to furnish by more complete 

 decomposition the large amount of silica now seen as secondary 

 quartz cementing the sand grains. But this is a problem which 

 can not be considered at present. 



As to the topograph}' of the land surface at the time of the 

 incursion of the Potsdam sea, but one conclusion is indicated by 

 the data at hand, though the evidence is cumulative, rather than 

 conclusive in separate instances. It is a very general rule that, 

 where the Potsdam and the crystallines are shown in close prox- 

 imity (and such localities are very numerous), the uneven surface 

 of the latter rises at many points far above the base of the 

 former; and, were the flat beds of the Potsdam to be extended, as 

 they once were, they would wrap around these knobs of the 

 crystallines. Again, high ridges of the crystallines often rise to 

 a considerable elevation above the top of adjacent horizontally 

 bedded areas of Potsdam. These latter cases, of course, differ 

 in degree, not in kind, from the foregoing. To account for these 

 phenomena only two explanations are available: either the Pots- 

 dam was deposited on a very uneaven surface, or it was deposited 

 on a flat surface, and subsequent faulting has produced the 

 existing relations. The latter explanation would require an im- 

 mense number of faults, running in every direction, and often 

 with very sinuous courses. As a matter of fact, only one fault, 

 and this a very small one three miles north of Redwood, has been 

 noted, and it is highly improbable that, of the large number re- 

 quired by this explanation, many would not be readily detected. 



In some cases, the Potsdam is so nearly in contact with the 

 uneven crystallines that a fault is hardly possible, while in one 

 instance there is a well defined inlier of crystalline in Potsdam, 

 which is of course, so far as it goes, conclusive evidence. 



