GEOLOGY OF LAKE PLEASANT QUADRANGLE 57 



and on a large scale in the case of the great depressed block 

 lying between the Elbow-Three Ponds Mountain and Dewey Creek 

 faults, which join or nearly join a few miles northeast of Griffin 

 (Stony Creek sheet). 



SUMMARY OF GEOLOGIC AND PHYSIOGRAPHIC 

 HISTORY! 



Precambric history 



The earliest known condition of the area of the quadrangle dates 

 back to the very ancient Grenville times 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 great masses of anorthosite, 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 

 intrusions. We have no definite knowledge concerning the topo- 

 graphy of this land mass when it was high above sea level, but we 

 do know that it underwent erosion for a vast length of time ex- 

 tending through the late Precambric and even into the early 

 Paleozoic. 



After the great intrusions of syenites and granites came the minor 

 intrusions of gabbro and diabase, the latter having been forced out 

 in late Precambric time as shown by the fine-grained and non- 

 metamorphosed character of the rock now near the surface. 



Paleozoic history ^ 



The long period of erosion above mentioned as beginning in the 

 Precambric extended to Potsdam time in the late Cambric period 

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

 rock surface was the Potsdam sandstone. Some thousands of feet 



1 Those not especially versed in the science of geology might do well 

 first to consult the writer's "Geological History of New York State" pub- 

 lished as N. Y. State Museum Bulletin i68, particularly chapters i to 4 

 inclusive. 



2 A recent paper by the writer rather fully discusses " The Early Paleozoic 

 Physiography of the Southern Adirondacks " in N. Y. State Mus. Bui. 

 164, 1913, p. 8o-94- 



