JO NEW YORK STATE MUSEUM 



Cambrian the case is different. The first deposit to form in the 

 late Cambrian sea was the Potsdam sandstone which is well repre- 

 sented in the St Lawrence, Champlain and Mohawk valleys, these 

 regions all having been submerged under the Potsdam sea. In the 

 southeastern Adirondacks the Potsdam sea certainly extended in 

 as far as Wells (southern Hamilton county), North River (nortii- 

 western Warren county), and Schroon Lake (southern Essex 

 county), because small outlying masses of Potsdam sandstone occur 

 in those localities, having been formerly connected with the larger 

 areas around the Adirondacks as above explained. The Potsdam 

 sea surrounded and more or less lapped over on the borders of the 

 Adirondack region, particularly the southeastern portion. There 

 is no evidence that the interior of the Adirondack region was sub- 

 merged, but rather it almost certainly formed a large island in the 

 Potsdam sea. 



Marine conditions continued with the deposition of alternating 

 layers of sandstone and dolomite upon the Potsdam. This is called 

 the Theresa formation. After still greater submergence, the impor- 

 tant formation known as the Little Falls dolomite was deposited 

 layer upon layer to a thickness of usually several hundred feet in 

 the comparatively clear waters of the latest Cambrian sea. The 

 Little Falls sea swept all around the Adirondacks. Occurrences 

 of the formation in the outliers at Wells (Hamilton county) and 

 at Schroon Lake prove that the Little Falls sea extended well over 

 the eastern Adirondack area, including much at least of the Schroon 

 Lake quadrangle. Map figure 6 graphically shows the approximate 

 relations of land and water during late Cambrian time. 



The Cambrian period closed with all of northern New York 

 above sea level, but early in the Ordovician period a submergence 

 set in, reaching a maximum about the middle of the period. Even 

 at the time of maximum submergence in the Middle Ordovician, 

 the best evidence points to the existence of a considerable island 

 comprising the interior of the Adirondack region (see map figure 

 6).^ Mid-Ordovician strata at Wells indicates the presence of the 

 sea of that age over southern Hamilton county. Though mid- 

 Ordovician strata are not exposed at or near Schroon Lake, such 

 rocks may be there concealed, or they may formerly have been 

 present. In any case, their strong development in the Champlain 

 valley only 15 or 20 miles to the east renders it highly probable 



^The early Paleozoic physiography of the southern Adirondacks is dis- 

 cussed by the writer in a paper in N. Y. State Mus. Bui. 164, p. 80-94. 

 1913- 



