﻿56 NEW YORK STATE MUSEUM 



The second line of evidence is that obtained in following and 

 mapping the long Potsdam boundaries. A single example, that of 

 the Potsdam margin along the west bank of Indian river in the 

 southeast corner of the Alexandria sheet and for 1 mile southward 

 on the Theresa sheet, will serve as well as a multitude of illustra- 

 tions to indicate what the evidence is. The section is convenient 

 since it has a horizontal base, the edge of the Indian river marsh. 

 The Potsdam faces the river in a prominent bluff which, when it 

 comes down to the marsh level, as it frequently does, forces the 

 pedestrian to take to the swamp, so that the walk is not recom- 

 mended" as a pastime. But the unbroken cliff margin renders accu- 

 rate mapping of the Potsdam base possible, and underneath it Pre- 

 cambric exposures are numerous-. At the south end of this section, 

 on the Theresa sheet, inspection of the map will show the Potsdam 

 coming down to the river level in a point. Going northward it soon 

 runs up the bank until the base is 40 feet above the river, with 

 Grenville limestone outcrops showing beneath, then it returns to 

 the river level and again rises, repeating the performance three times 

 within a mile of distance. Soon after passing on to the Alexandria 

 sheet the sandstone retreats prominently up the bank and back from 

 the river, showing a 60 foot thickness of limestone underneath, then 

 returns to marsh level, withdraws 30 feet up the bank, comes back 

 again forming a point, retreats quickly for 60 feet up the bank and 

 again returns to the river, all the while with limestone underneath, 

 cut by occasional granite dikes, so that all these oscillations merely 

 represent irregularities of the limestone surface. Northward from 

 this last point of reaching the river, however, the limestone is cut 

 out by granite gneiss, and this turns the Potsdam straight up the 

 bank and out to the road, with a rise of more than 100 feet in the 

 level of the Precambric surface. Equally striking are the oscilla- 

 tions in level of the same margin when followed southward on the 

 Theresa sheet, and this margin is easy to follow, using the railroad 

 as a base. There are many other excellent examples, since this sort 

 of thing is the rule throughout the district. The mapping of the 

 Potsdam base is thereby rendered laborious but nothing can be 

 imagined more beautifully demonstrative of the character of the 

 surface on which the Potsdam rests and its identity with that of 

 the surface from which the Potsdam has been removed. 



Lastly there is the evidence given by the actual contacts. There 

 are quite a number of these, more than the writer has seen in the 

 entire remaining border of the Adirondack region. Besides the 

 actual contacts there are a host of others where but a few feet of 



