136 NEW YORK STATE MUSEUM 



of the basal portion of the sediments and caused the complete dis- 

 appearance of their old floor of deposit. 



There followed a very long period of erosion of the region during 

 which it was above sea level. A great thickness of rock was worn 

 away from the surface, bringing to daylight the tops of the great 

 igneous masses which originally solidified much below the surface. 

 The final effect of the long erosion period was to have reduced the 

 entire region to one of low altitude and small relief. Toward the 

 close of this erosion period renewed igneous activity resulted in 

 the formation of the trap dikes. 



Paleozoic history. The Adirondack region then developed a 

 tendency to dome upwards centrally and to sag at the margins. 

 These sagging margins passed from time to time beneath sea level 

 and received accumulations of deposit. In these troughs the early 

 Paleozoic sediments of the region were laid down. Oscillations 

 of level were of frequent occurrence, accompanied by warping of 

 the region. Submergence in the different troughs alternated; it 

 was the rare exception that all were depressed at the same time. 

 In this report the deposits of the Champlain-Hudson trough (the 

 so-called " Chazy basin ") chiefly concern us. 



To the eastward of the Chazy basin downwarping developed 

 one or more other troughs, in which deposits quite different from 

 those of the Chazy basin were laid down. Since their formation 

 these rocks have been overthrust to the west upon the rocks of the 

 Chazy basin, and directly adjoin them upon the east. 



CAMBRIC HISTORY 



Chazy basin deposits. Potsdam sandstone. The first Pale- 

 ozoic deposit of the Adirondack margins was the Potsdam sand- 

 stone, an accumulation of coarse, quartzose sands and gravels. The 

 accumulation began first on the northeast, in Clinton county, and 

 extended itself progressively to the west and to the south. Only 

 the upper portion of the formation is found in the Saratoga region. 



This upper portion contains marine fossils and must have been 

 laid down in shallow marine waters. But marine fossils are lacking 

 in the lower half of the formation, in Pinton county, and many 

 of its characters suggest that it was laid down above sea level 

 instead of below it. They suggest also that the climate was arid. 

 But whether marine or not, the accumulation began because the 

 district sagged down. This sagging began at the north and slowly 



