32 NEW YORK STATE MUSEUM 



hibited in contact with each other. Generally the older rocks have 

 been worn down to an undulating plain (or peneplain), and the 

 succeeding beds made from the fragments which were worn from 

 the old land. 



In the area under consideration, the ancient erosion surface of 

 pre-Cambric rocks was overspread by a deposit of sand and occas- 

 ionally gravel, which commonly possesses characteristics pointing 

 to a very local origin. Thus the pebbles found in the lowest layers 

 of the covering sands, i. e. the Potsdam sandstone, are sometimes 

 of the same lithic character as the crystalline rocks near by. 

 The Potsdam sandstone is a shallow-water rock, and during its 

 accumulation a progressive subsidence of the sea floor took place, 

 thus allowing the deposition of beds of considerable thickness. 

 This subsidence brought with it a northward migration of the shore 

 line of the sea, so that the region of the former coast line gradually 

 became more remote from the shore. As a consequence, land- 

 derived material became less abundant in this ofif-shore district, 

 being deposited mainly along the new coast line, while farther out 

 to sea calcareous deposits, resulting in part from the shells of organ- 

 isms, became relatively more abundant. A profile through the 

 strata of this region, such as would be obtained in a well or shaft 

 sunk to the crystalline floor, would show a progressive decrease in 

 the land-derived, or terrigenous material from the Potsdam sand- 

 stone upward to the top of the Trenton limestone, and a correspond- 

 ingly progressive increase in the amount of calcareous matter. 

 This indicates a sustained subsidence of the sea floor, and hence a 

 migration of the shore with its attendant terrigenous deposit. It 

 will also be seen that the lithic character of any particular formation 

 is not the same throughout its extent, but that the local characteris- 

 tics, or fades, show considerable variation. Close to the shore each 

 formation would present a terrigenous character, i. e. would show 

 gravelly, sandy or clayey facies, while away from shore each forma- 

 tion would pass into its calcareous facies, which would increase in 

 purity with the increase in distance from the source of supply of 

 terrigenous sediment. Thus the Potsdam formation has calcareous 

 as well as sandy facies, with facies of intermediate type connecting 

 them. 



