58 NEW YORK STATE MUSEUM 



more than one advance and retreat of the great glacier, as was the 

 case in the upper Mississippi valley, it is easy to understand how 

 the old divides could have been cut down during one or more inter- 

 glacial stages by deflection of the rivers from their preglacial 

 courses. All known facts harmonize with this explanation. Stol- 

 ler^2 has come to a similar conclusion in regard to the cutting of 

 the gorge of the Hudson across the preglacial mountain barrier east 

 of Corinth. 



SUMMARY OF GEOLOGICAL HISTORY 

 Precambrian History 



The earliest known condition of the quadrangle dates back to the 

 very ancient Grenville time of the Archeozoic era when ocean 

 water covered all of northern New York as well as vast adjoining 

 areas. That this condition prevailed for a long time (at least a 

 few million years) is proved by the great thickness of sediments 

 which were deposited in that ocean. 



Next, the bodies of gabbro, syenite and granite were intruded 

 into the Grenville strata. Also there was metamorphism of the 

 rocks and a general elevation of the whole Adirondack region well 

 above sea level, probably at or near the time of the igneous in- 

 trusions and without notable folding of the rocks. We have no 

 definite knowledge concerning the topography of this land mass, 

 but we do know that it underwent erosion for a vast length of time 

 extending through late Precambrian time and even into the very 

 early Paleozoic. 



After the great syenite-granite intrusions came the minor in- 

 trusions of diabase and possibly of some gabbro, the former having 

 been forced into fissures to form dikes in late Precambrian time as 

 proved by the fine-grained texture of the rock. 



Paleozoic History 



The long period of erosion above mentioned as beginning in the 

 Precambrian extended to Potsdam time in the late Cambrian period 

 as proved by the fact that the first deposit upon the Precambrian 

 rock floor was the Potsdam sandstone. Some thousands of feet of 

 Precambrian rocks materials must have been removed during this 

 erosion interval because the Precambrian rock structures immedi- 

 ately below the Potsdam could have been formed only at very con- 



N. Y. State Mus. Bnl. 183, 1916, p. 31. 



