86 NEW YORK STATE MUSEUM 



southeastern border of the Adirondacks and in the outHers at 

 Schroon Lake and Wells shows that the Little Falls sea extended 

 over at least as much of the southern Adirondacks as did the 

 Potsdam-Theresa seas. In the Mohawk valley region it ex- 

 tended considerably farther westward overlapping upon the Pre- 

 cambric rock to the southwest corner of the Wilmurt quadrangle 

 where the dolomite thins out to disappearance. From this point 

 northwestward the Precambric rock margin shows no dolomite, 

 thus proving the absence of the Little Falls sea there. The very 

 rapid decrease in thickness of the dolomite from four hundred 

 feet at Little Falls to complete disappearance just beyond the 

 northern boundary of the Little Falls quadrangle also shows the 

 limit of the sea in that district. Accordingly there must have 

 been a large land mass in the southwestern Adirondack region. 

 The very presence of so many sand grains in the dolomite (giv- 

 ing rise to the old name Calciferous sandrock) requires that it 

 was deposited comparatively near a land mass. Thus during 

 late Little Falls time the eastern portion of the southern Adiron- 

 dacks was submerged while the western portion remained dry 

 land, the shore line extending from the southwest corner of the 

 Wilmurt quadrangle most probably in a northeasterly direction 

 through the southern Adirondacks. That its shore line was, in 

 general, a little farther westward than that of the Potsdam sea is 

 strongly suggested by the fact that all the Cambric sediments 

 were gradually accumulated in a downsinking trough occupying 

 the southeastern Adirondack area. This idea of a gradual west- 

 erly encroachment of the Cambric sea is borne out by the follow- 

 ing facts : The thickness of the Cambric section within the Sara- 

 toga quadrangle is from four hundred to five hundred feet ; 

 within the Broadalbin quadrangle near Northville four hundred 

 to four hundred and lifty feet ; and at Wells about two hundred 

 feet. This rapid decrease in thickness of two hundred feet from 

 Northville to Wells within a distance of fourteen miles shows a 

 westward to northwestward encroachment of the Cambric sea 

 and that the downward slope of the surface here receiving Cam- 

 bric sediments was fourteen feet a mile toward the southeast. 

 According to Ulrich and Cushing**^ there is a distinct strati- 

 graphic break represented by a notable erosion unconformity at 

 the top of the Little Falls dolomite. Thus all available evidence 

 supports the idea that, by the close of the Cambric period, subsidence 



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