l8 NEW YORK STATE MUSEUM 



times the evidence seems conclusive that Grenville gneisses 

 have been melted and actually assimilated by the molten intru- 

 sions so that various rocks of intermediate character have 

 resulted. 



Only a few diabase and gabbro dikes have been found. Several 

 of the diabase dikes are distinctly porphyritic with large plagio- 

 clase crystals, but the exact nature of these rocks has not yet 

 been determined. 



The chief geologic interest of the quadrangle centers about 

 the valley at Wells because of the location there of the important 

 outlier of Paleozoic rocks comprising Potsdam sandstone, 

 Theresa passage beds. Little Falls dolomite, Black River (Low- 

 ville) limestone, Trenton limestone, and Canajoharie (Trenton) 

 black shale. Altogether the thickness of these strata is about 

 five hundred feet and their areal extent about three square miles. 

 The valley is of the nature of a fault basin with distinct faults 

 along the eastern and western sides and a minor one between. 

 Along the western side of the outlier the displacement of the 

 fault is no less than 2000 feet, the Canajoharie shales showing a 

 decided updrag effect near the fault. A very careful survey of 

 the vicinity of Wells has been made resulting in the first detailed 

 areal map (with structure sections) of this the most interesting 

 Paleozoic rock outlier in the Adirondacks. 



Another feature of special interest is the discovery of an out- 

 lier of Paleozoic rock in the Sacandaga valley from one to two 

 miles above Hope post office. The only strata visible are con- 

 siderable ledges of Little Falls dolomite and a little of the ' 

 Theresa passage beds and Black River limestone. These strata 

 are sharply dowmfaulted at least 1200 feet against the steep 

 mountain on the western side of the valley. A minor fault 

 appears to bound this outlier on the east so that this too seems 

 to be a fault basin. 



The major topographic features of the quadrangle are largely 

 determined by normal faults, most of which strike northeast- 

 southwest, though certain important cross faults also occur. 

 There are many good examples of fault blocks, ridges and 

 basins. 



Glacial striae show the movement of the great ice sheet to 

 have been southward to southwestward. There are several fine 

 examples of extinct glacial lakes, especially those in the valley 

 at Wells ; alon^ the Sacandaga river between the mouth of the 



