MUSEUM OF COMPARATIVE ZOOLOGY. 323 



When the attempt is made to restore the geographic changes by which 

 the several strata were made to vary, it cannot be denied that this con- 

 formity is difficult to understand. The sands of the Hudson River group 

 indicate a return of the shore line on the east after the open ocean con- 

 ditions of the Trenton formation ;* the shallowing of the waters and the 

 westward advance of the shore continued certainly as far as the present 

 line of the Hudson. If the unconformity at Becraft's Mountain be ac- 

 cepted, then we may follow the generally allowed belief in the folding of 

 these sandstones accompanied by their elevation and erosion ; and the 

 veiy variable composition of the Niagara group as a whole, and the prob- 

 able changes of level during the Onondaga (Salina) period are most likely 

 to be explained by the oscillations of the adjoining land on the east dur- 

 ing the Green Mountain growth. Excepting the few feet of beds found at 

 the northern end of the Quarry Hill, which were not definitely referred 

 to any group, there is nothing to be seen in the Catskill section to repre- 

 sent this vast lapse of time ; for directly and conformably above the Hud- 

 son River sandstones come the Lower Helderberg limestones, which mark 

 the second and greater eastward advance of the sea in the Upper Silurian, 

 the first being that of the Xiagara limestones. The present Catskill 

 district cannot then have been dry land, for it shows no eroded surface 

 beneath the Lower Helderbergs so far as we could discover. It could 

 hardly have been far under water, for then it should have received some 

 share of the various sediments so plentifully supplied to the ocean far- 

 ther west and southwest. It may, therefore, be best to suppose that our 

 district lay just off shore, or almost between wind and water, and either 

 received very little detritus, or else was alternately covered and swept 

 bare again by shoal water currents, so that in the end it had gained 

 scarcely any rock material. 



The change that followed next was not so much a depression of the 

 ocean floor as a distant eastward retreat of the shore line ; for the Lower 

 Helderberg limestones show shallow waters, with freedom from shore 

 sediments such as the Green Mountain rocks could have furnished. The 

 Catskill shaly limestone probably marks a slight departure from this 

 open ocean, and the presence of a neighboring shore, for it contains 

 much more non-calcareous material than the limestones above and be- 

 low it. 



This second oceanic cycle ends with the Oriskany sandstone, marking 

 a return of the shore, though perhaps not a very near approach ; and 



* See Professor Newberry's "Circles of Deposition," &c. Airier. Assoc. Proc., 

 XXII., 1873, (2,) 185-196. ' 



