SIXTH REPORT OF THE DIRECTOR ICjOO, 139 



hall, N. Y., in central Pennsylvania and in northern Virginia, leave 

 little doubt that this sea extended down the western side of the 

 Appalachian valley to the Mississippi valley where the same species 

 are found in the middle formations of the Ozarkic system. 



A thinned edge of the upper Potsdam runs westward into the 

 Mohawk valley, but thins out to zero rather rapidly, letting the 

 Little Falls dolomite down on the Precambric. It is thought that 

 along the Mohawk line the Potsdam shore had a southwesterly 

 trend, or rather a trend more to the south than the present Pre- 

 cambric margin, the two meeting at an angle; east of the meeting 

 point the Potsdam appears underneath the Little Falls, while west 

 of it the Potsdam is either absent or erosion has not yet cut down 

 to it. This involves the assumption that the Little Falls subsi- 

 dence covered more of the southern part of the old land area than 

 did the Potsdam, so that, within the Potsdam zone there would be 

 a strip of territory with the Little Falls resting on the Precambric. 

 In some localities there is direct evidence that this actually oc- 

 curred, and it was likely true of much of the southern and eastern 

 parts of the Adirondack border. 



In the Thousand islands region we find merely the thinned edges 

 of the marine Potsdam and Theresa formations with no sign of 

 the Little Falls dolomite. However, since these were laid down 

 in the extreme westerly portion of the St Lawrence trough it is 

 theoretically possible that their deposition occurred while the dolo- 

 mite was being deposited along the Champlain and Mohawk lines. 

 But there is no positive evidence that such a condition obtained. 

 On the contrary, according to the trend of the scant faunal evidence 

 and the probabilities suggested by general disastrophic considera- 

 tions the lower or typical Theresa on the west flank of the Adiron- 

 dacks is essentially contemporaneous with the transition beds on the 

 east side; and deposition was prohibited by emergence on the west 

 side when the Little Falls was being laid down in the Champlain 

 and Mohawk valleys. 



Following the Little Falls deposit, warping and differential up- 

 lift ensued, causing the shore lines to retreat from the district and 

 resulting in the unconformity at the top of the Little Falls. There 

 was some wear also since the summit is uneven and the Tribes 

 Hill rests on different beds of the Little Falls; and, the returning 

 waters assumed a different arrangement, with a more diversified 

 shore line. 



Beekmantown depression commenced with the Tribes Hill de- 



