REPORT OF THE DIRECTOR I912 85 



through the village of St Johnsville in the Mohawk valley. Thus 

 we are certain that the shallow Potsdam sea overspread prac- 

 tically the whole southern Adirondack region east of this line ex- 

 cept for a few local knobs or ridges of hard Precambric rock 

 which remained above the sea level. A fine example of such a 

 local projection above the Potsdam sea level has been described 

 by the writer^^ in his report on the Broadalbin quadrangle. That 

 the Potsdam shore line extended a short but unknown distance 

 farther west than Wells and North River is certain because a 

 considerable thickness of sandstone is still represented at those 

 places. This conclusion regarding the position of the Potsdam 

 shore line is in harmony with the statement of Ulrich and 

 Gushing^" wdien they say : " It is thought that along the Mo- 

 hawk line the Potsdam shore had a southwesterly trend more 

 to the south than the present Precambric margin, the two 

 meeting at an angle ; east of the meeting point the Potsdam ap- 

 pears under the Little Falls, while west of it the Potsdam is 

 either absent or erosion has not yet cut down to it." 



That the southwestern Adirondacks were not submerged un- 

 der the Potsdam sea is proved by the complete absence of the 

 sandstone from the southwestern border; the very character of 

 the sediments (sands and pebbles) which demands nearness to a 

 mass of Precambric rock ; and the negative evidence from the 

 fact that no outliers of Potsdam have ever been found in this re- 

 gion. The Potsdam sea did extend up the St Lawrence valley as 

 shown by the presence of the sandstone there. 



Thus we conclude that a long, low, land area of Precambric 

 rock extended in a northeast-southwest direction through the 

 Adirondack region, and that this height of land in Potsdam time 

 had almost exactly the same position as the present main axis of 

 elevation of the mountains. 



Since the Potsdam sandstone grades into the succeeding, alter- 

 nating sandstone and dolomite beds of the Theresa and the two 

 formations have almost precisely the same distribution, we are 

 safe in asserting that the physical geography conditions of The- 

 resa time were essentially like those of Potsdam time except that 

 the southeastern Adirondacks were then even a little more sub- 

 merged. 



The distribution of the Little Falls dolomite which succeeds 

 the Theresa beds without unconformity along the southern and 



10 Pages 51-52. 

 16 Page 139. 



