390 PALJEOZOIC TIME. 



The Trenton spread over both, with perhaps the exception of the Green 

 Mountain portion ; and, throughout, it was almost solely limestone. 



The Hudson formations cover both, with the same exception as in the case of 

 the last ; but they are shales in the East, and limestones mainly in the West. 



The preceding are the universal formations. Passing now to the Upper 

 Silurian : — 



In the Niagara period, the Oneida conglomerate occurs in central New York 

 and along the Appalachians ; the Medina beds have nearly the same position, 

 but also spread over Canada, west of New York, and are sandstones and marls; 

 the Clinton beds have approximately the same eastern distribution, but spread 

 farther to the west along the northern portion of the Interior basin, reaching to 

 Wisconsin,— and, while mostly arenaceous in the East, they are in general calca- 

 reous in the West; the Niagara limestone has again the same position over 

 central- New York and the Appalachians, but extends over nearly the whole 

 Interior basin, showing an expansion or change in the interior sea; and its 

 rocks, excepting those of the Appalachians, are mainly limestones. 



In the Salina period, there is the same extension of the formation from the 

 Appalachians north over central New York, but to the west it spreads only along 

 the north, where it reaches to Iowa ; the rocks on the east are marls and sand- 

 stones, and those of the West mostly limestone. 



In the Lower Hehlerberg period, the rocks stretch from central New York 

 southwestward along the Appalachians, and do not spread much to the west of 

 New York and Canada. The rock is a limestone, — the first of any extent in the 

 Appalachians of Pennsylvania since the Trenton. 



This extension of the Appalachian strata northward over central New York, 

 and their maximum thickness there for those latitudes, have been accounted for 

 on the supposition that the Green Mountain region became a part of the com- 

 paratively permanent dry land after the close of the Lower Silurian (pp. 228, 243), 

 and, consequently, the oscillating area was transferred a little farther to the 

 westward. It, however, did not reach north to the Azoic of New York, which 

 was still a portion of the stable part of the continent. 



In the Devonian age : — 



The Oriskany formation was mainly confined to central New York and the 

 Appalachians, but some thin beds occur in the Mississippi basin : the rocks are 

 mostly sandstones. 



The Upjier Helderberg was the coral-reef period, and limestones were spread 

 over the larger part of the Interior basin, and also over the Appalachians. 



The Hamilton beds were extensive in central New York and along the Appa- 

 lachians, but very thin to the westward over the interior : the " black slate" of 

 the Genesee epoch is its thickest portion, and this is remarkable for its wide 

 distribution over the Interior basin, and its persistent uniformity of character, — 

 which proves that it must have been formed in waters of very uniform depth, 

 either very shallow or somewhat deep, — probably the former. 



The Chemung and Portage beds have great thickness in southern New York 

 and the Appalachians, and are very thin to the west, and in many parts want- 

 ing : they are sandstones. 



In the Carboniferous age : — 



The Subcarboniferous formations are thousands of feet thick in the Appa- 



