GEOLOGY OF THE PORT LEYDEN QUADRANGLE 39 



feet of mostly sandstone. Orton refers all of these sandstones to 

 the Potsdam. The bottom of the section at Stillwater is : 



Feet 



Dark Trenton 370 



Sand and shales 40 



Sand, green and white 25 



Black limestone 6 



Red and white sandstone, calcareous 18 



Precambric struck at 1697 



Fossils in the black limestone layer prove the Upper Cambric 

 (Potsdam) age of the deposit. The presence of this Potsdam in 

 Oswego county and its absence along the Paleozoic-Precambric 

 boundary to the eastward on the Port Leyden sheet, affords con- 

 clusive evidence of overlap. 



Again, we have a strong argument in favor of overlap if we con- 

 sider the whole thickness of sediments between the top of the Tren- 

 ton and the Precambric. Details will be presented later, but suffice 

 it to say now that a comparison of the thickness of these sediments 

 in Oswego county with those near Port Leyden shows a thinning 

 of several hundred feet in passing toward the latter place. Such a 

 marked diminution in thickness toward the northeast and east is 

 just what would be expected in the case of overlap. 



SURFACE OF THE PRECAMBRIC ROCKS 

 Smoothness of the surface v^^hich received Paleozoic deposition 



A study of the Paleozoic-Precambric line of contact gives strong 

 evidence in favor of the statement that the sinking surface which 

 received Paleozoic sedimentation must have been worn down to a 

 remarkably smooth condition (peneplain). Except for a few miles 

 near the southern edge of the map, the Paleozoic-Precambric 

 boundary line can be drawn with a considerable degree of accuracy. 

 A glance at the geologic map will show that this boundary line is 

 a very regular one which at no point shows any rapid elevation or 

 depression. Such a regular line of contact is precisely what one 

 would expect where sediments have been laid down upon a smooth 

 floor and then, after elevation, have been stripped off rather regu- 

 larly by erosion. Even a comparatively small elevation or depres- 

 sion along the contact line could be recognized. At several points 

 where the actual contact is exposed, the Precambric floor appears 

 to be smooth. 



