GEOLOGY OF THE NORTHERN ADIRONDACK RBGIOIN 279 



of red color and have the composition of syenites and granites. 

 The latter are somewhat the older. The northeastern Adaron- 

 dacks were the main scene of this igneous activity. The red dikes 

 are practically confined to Clinton county. The black dikes are 

 much more numerous and have a much wider range, but are vastly 

 more abundant in Clititon and Essex counties fthan elsewhere. 



Erosion still continuing after the close of the igneous activity, 

 all vestiges of the surface volcanics disapppeared, along with a 

 thickness of rocks of considerable amount, but to be measured in 

 hundreds, rather than in thousands of feet. 



Early Paleozoic history 



The long period as a land area finally came to an end, a move- 

 ment of subsidence was initiated in the region and it became 

 depressed, slowly passed beneath the level of the sea and began to 

 receive deposit on the rather evenly worn surface, the valley bot- 

 toms necessarily passing beneath the sea before the low divides 

 were overtopped. This subsidence began at the northeast and 

 slowly progressed southwestward. As zone after zone came 

 within reach of the cutting of the waves, they would tend 

 to pare it away to a sm'ooth surface, and in parts of the region 

 this Avave action may have been a considerable factor in evening 

 it. As zone after zone passed beneath the sea, deposit would com- 

 mence on the surface, and, as the subsidence began on the east 

 and northeast and progressed westward and southwestward, de- 

 posits of progressively younger and younger age appear resting on 

 the old land surface in passing from east to west, producing what 

 is called overlap. Because of this, the earliest paleozoic deposits, 

 those of Lower and Middle Cambric age, do not appear about 

 the Adirondack region at all, though found not many miles to the 

 eastward, where they were deposited in a separate basin. Be- 

 cause of this, the Potsdam formation, or Upper Cambric age, 

 does not appear on the west and south of the Adirondack region, 

 though in great force on the north and east. 



Potsdam formation. This coarse, often pebbly, massive sand- 

 stone deposit was the first formation laid down on the old land 



