Handbook of Paleontology 239 



raised the northern and western flanks of the island 

 restricting the sea to the Champlain trough in the east 

 and the Mohawk basin on the south. The Potsdam 

 sandstone (Emmons '38) was named from typical ex- 

 posures in the vicinity of Potsdam in St Lawrence 

 county. The most extensive development occurs in 

 the northern part of the state bordering the Adiron- 

 dacks ; the areas on the eastern and southern bounda- 

 ries of these mountains are less extensive and discon- 

 tinuous. Sections showing a representation of all the 

 formations from the Potsdam through the Little Falls 

 dolomite may be seen at Ticonderoga, Whitehall, Sara- 

 toga Springs and in the Mohawk valley at Cranesville, 

 Tribes Hill and Broadalbin. Southward from Sara- 

 toga the Potsdam sandstone rapidly thins and disap- 

 pears, probably due to overlap of later beds. The sand- 

 stone everywhere abounds in ripple marks showing its 

 shallow-water or near-shore origin. It constitutes all 

 the rock in the walls of the famous Ausable Chasm. 

 The Potsdam is a light-colored, vitreous sandstone 

 carrying occasional calcareous layers in the upper por- 

 tion and with more or less coarsely conglomeratic beds 

 at the base. The thickness is variable because of the 

 irregular erosion surface of the Precambrian beneath 

 but from sections and borings the maximum thickness 

 has been estimated as over 1500 feet. In its lower 

 portion it was probably a continental deposit but the 

 upper portion carries a marine fauna which continues 

 through the Theresa passage beds into the Little Falls 

 dolomite. 



The Theresa formation (Cushing '08) received its 

 name from exposures in the town of Theresa, Jeffer- 

 son county, and as first described included the passage 



