GEOLOGY OF THE NORTHERN ADIRONDACK REGION 385 



On the south and west sides of the region, the Utica shales are 

 overlain conformably by a group of shales and sandstones, often 

 with passage beds between ; and the group has usually a large 

 thickness, as the section and well records just quoted demonstrate. 

 There is no direct evidence that equivalent rocks were ever depos- 

 ited in the Chazy basin of the Champlain valley, but neither is 

 there any weighty evidence that they were not. South of the 

 Mohawk, however, and all along the west side of the region, they 

 appear in force. The above quoted records show that the forma- 

 tion thins westward through the Mohawk valley, is thinnest at 

 Utica, where only its base is present, and thence thickens rapidly 

 to the north and west. Walcott has given a thorough discussion 

 of the evidence, showing that it argues for a shallowing of the sea 

 along the Utica meridian early in Lorraine times, thinning the 

 section there, and preventing thereafter a commingling of the 

 western (Lorraine) fauna, with the forms to the eastward of the 

 barrier, which hence separated the eastern Mohawk basin from 

 that of the interior. 1 It is by no means improbable that this 

 uplift at Utica is but part of a greater movement, which extended 

 thence to the northeast, bringing much, if not most of the Adiron- 

 dack region above sea level and causing also cessation of deposi- 

 tion in the Chazy basin. 



The great thickness of the Lorraine rocks, both in the eastern 

 Mohawk region and in Jefferson and Oswego counties, together 

 with the fact that their present line of outcrop is owing to long 

 continued, surface erosion, and that the effect of this erosion is 

 to cause the line of outcrop continually to recede from the Adiron- 

 dacks, sufficiently indicates that in the past they must have ex- 

 tended in over them, likely for several miles, and that in some 

 considerable thickness; and that during Lorraine times a consid- 

 erable area, specially on the northwest and the southeast, 

 remained yet submerged, in spite of the uplift described above. 

 In addition, it is by no means impossible that some of the later 

 Siluric rocks may have overlapped on the southern and western 

 margins of the region, though this is much more open to question 

 than is such a former extension of the Lorraine rocks. Certainly, 

 the general tendency to subsidence over the district, initiated in 



^eol. Soc. Am. Bui. 1:344-50. 



