GEOLOGY OF THE VICINITY OF LITTLE FALLS 63 



interrupted at the close of the Beekmantown conditions. There 

 is little evidence that the subsiding movement was changed to one 

 of elevation, except locally about Canajoharie, where it would 

 seem that a slight arching of the surface occurred, accompanied 

 by some slight erosion, before the downward movement was re- 

 sumed late in the Trenton. Otherwise conditions seem best ac- 

 counted for on the assumption of a check to the downward move- 

 ment, which Avas thence forward in very slight amount for a time, 

 with many small local variations. With cessation of subsidence 

 deposition must soon cease and the evidence of local variations is 

 convincing. The field evidence led the writer to the belief in an 

 unconformity at this horizon, before the search through the litera- 

 ture made him aware that others had brought out evidence along 

 the same line. 



Absence of the Chazy formation in the Mohawk valley 

 One effect of this pause in subsidence (with perhaps slight ac- 

 companying uplift) was to effect an entire separation between 

 the basin of Mohawk valley deposition and that of the Champlain 

 valley, for the time being. In the latter we have a great forma- 

 tion, the Chazy limestone, with a maximum thickness of 800 feet 

 in Clinton county, interposed between the Beekmantown and the 

 Trenton. This formation rapidly diminishes in thickness when 

 followed to the southward in the Champlain valley and wholly dis- 

 appears before the Mohawk is reached. Its thinning and disap- 

 pearance seem to be due to a progressively diminishing rate of 

 subsidence toward the south, rather than to overlap. This point 

 will be again reverted to ; and, if well taken, it follows that, while 

 a considerable downward movement, progressively greater toward 

 the north, was taking place in the northern district, little or no 

 subsidence, and hence an almost complete interruption of deposi- 

 tion, characterized the southern region during the time interval 

 represented by Chazy deposition. 



Sudden thickening- of the Trenton westward 

 At Trenton Falls the Trenton limestone has a measured thick 

 ness of 270 feet, which must be increased by an unknown but small 

 amount, since neither the base nor the summit shows in the sec- 



