GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



380 



commenced on all sides of the Adirondack region: but, as was the 

 case also in the Potsdam, it was most pronounced on the north- 

 east and diminished in amount toward the west and south. The 

 formation must have encroached on the Adirondack island on all 

 sides, greatly diminishing its previous area. On the south side 

 specially a large transgression of the sea on the former land took 

 place, since the Potsdam shore line had been an unknown dis- 

 tance to the south of the present line of the Mohawk valley, while 

 •the Beekmantown has a thickness of from 300 feet to 500 feet 

 there, and its shore must have lain several miles to the northward. 

 On the west side of the region but little subsidence took place, 

 and the Beekmantown shore line lay to the west of the present 

 Precambric border there. In the Champlain region the forma- 

 tion has treble the thickness that it has along the Mohawk, and 

 the transgression of the sea on the northeastern portion of the 

 region must have been of vast extent. The Adirondack land mass 

 must certainly have been an island during the Beekmantown. 

 whose area was small compared with the present size of the 

 region, and which lay mainly in its western portion, extending 

 eastward for an unknown distance, greatest on the south side. 



Then conditions changed, and the downward movement was 

 replaced by an upward one, which caused cessation of deposition 

 on the south and west sides of the area, brought a large but 

 unknown amount of the previously submerged tract above sea 

 level, so that the Beekmantown island was greatly extended in 

 those directions, and shut off communication between the basin 

 on the southwest and that on the northeast. The latter district 

 did not feel the upward influence, but continued to subside, and 

 the limestones and dolomites of the Cassin formation were depos- 

 ited on the normal Beekmantown. The abundant fauna found 

 fossil in these beds and absent from the Beekmantown beneath, 

 must have entered the basin from the east or north, arguing for 

 extended dej>ression and open sea connection in one or the other, 

 or both, directions. 



The depression on the northeast persisted throughout Chazy 

 time, though likely with interruptions, of which the most impor- 

 tant is indicated by the basal sandstone of the Chazy. Like the 

 preceding Cassin subsidence, this diminished rapidly in amount 



