144 NEW YORK STATE MUSEUM 



(The Precambrian peneplane) 



10 Through cracks in this plain, the diabase dikes forced their 

 way upward toward what may have been active volcanoes on the 

 surface. 



(Age, probably Keweenawan, the top of the Precambrian, or the 

 base of the Cambrian) 



1 1 A period of erosion followed that removed the surface evidences 

 of volcanic action, if such existed. 



(The pre-Potsdam peneplane) 



12 Then came submergence of the land, followed by 



13 The deposition of the basement bed of the Potsdam sandstone, 

 which may have been of continental or desert origin, as contrasted 

 with sedimentaries formed under water. (The " Ausable " sand- 

 stone of some authors.) 



14 The land sank below the surface of the encroaching sea, in 

 which occurred 



15 The deposition of the marine or " white Potsdam " sandstone. 

 (The Keeseville sandstone of many.) 



(Age, Cambrian) 



16 The later limestones were next deposited. (The Beekmantown 

 limestone, the Chazy group, the Lowville limestone, the Black River 

 limestone, the Trenton limestone, and finally the Utica shale.) 



(Age, Ordovician) 



17 Regional uplift that raised the sediments above water. 

 (Age, Taconic uplift) 



18 Succeeding erosion removed most of the above sediments from 

 the central Adirondacks. 



19 A series of uplifts affecting the New England region much 

 more than the Adirondacks, nevertheless causing block faulting to 

 take place on a large scale throughout the region, especially in the 

 east, the downward displacements being to the east. The amount 

 of displacement was often very great, several thousands of feet in 

 some cases. 



(Time, Devonian through the Permian) 



20 During the period of uplift occurred the intrusions of the 

 later syenitic (trachytic) dikes in the Champlain district. 



21 Erosion continuing, produced in, time the "Cretaceous" 

 peneplane. 



22 Then the last continental ice sheet came from the north, 

 rounding the hills, modifying the drainage and destroying all vege- 

 table and animal life. 



(Age, Pleistocene) 



