GEOLOGY OF THE NORTHERN ADIRONDACK REGION 387 



has been treated in some detail in the foregoing pages, but a 

 concise summary of it may well find a place here. 



The Potsdam is confined mainly to the north and east sides of 

 the region. It comes around into the Mohawk region, but is thin 

 there and fades out to a vanishing point about midway of the 

 valley. It does not appear at all on the west side of the territory. 

 It is thickest on the northeast, in Clinton county, and there alone 

 is any great thickness of its peculiar, basal portion to be found. 

 To the south and west the formation thins by disappearance of 

 this base, and it would seem therefore that deposition must have 

 commenced on the northeast and advanced progressively westward 

 and southward, so that by the close of the Potsdam the north- 

 eastern district had undergone large submergence, whereas on the 

 southwest the shore line was yet outside of the present Precam- 

 bric margin, and the amount of subsidence had been trifiiug; 

 that is, that there existed a large, unsubmerged area on the south 

 and west at the close of Potsdam time. 



The formation was laid down on a comparatively even floor of 

 older rocks, whose evenness was mainly due to previous protracted 

 wear on it while a land surface ; but, in spite of the comparative 

 evenness, the floor shows much minor irregularity, whose amount 

 seems to increase with increasing thickness of the overlying Pots- 

 dam, All the workers on the north and east sides of the region 

 have observed and commented on the irregularity of the floor, 

 which sometimes amounts to some hundreds of feet. In the 

 Mohawk valley region the floor seems to have been exceedingly even 

 and flat, much more so than on the north. Since the former was 

 barely, or not at all submerged by the Potsdam sea, while the 

 latter was early invaded by it, the one did and the other did not 

 experience subaerial erosion during Potsdam time, this furnishing 

 an obvious reason for greater smoothness in the former, though 

 it may not be the whole reason. 



The upper division of the Potsdam would seem plainly to be a 

 marine sand deposit. Quite likely this is true of the middle 

 division also, though it is not so certain because of lack of fossils. 

 It seems possible that the basal portion, which is developed only 

 on the northeast, may represent a flood plain deposit under condi- 

 tions of climatic aridity. The red color and the undecayed char- 



