UPPER SILURIAN. 251 



land, and that was mainly to the north. It had enlarged somewhat 

 since the Lower Silurian era ; hut the greater part of the United 

 States was yet to be completed, by the deposition of the Devonian, 

 Carboniferous, and later beds. 



Shales and sandstones prevailed in the East, from the vicinity of 

 the Archtean of New York southwest along the Appalachian region. 

 But, in the west, the rocks of the Upper Silurian are mainly limestones ; 

 for the Niagara limestone is widely distributed in the Interior basin ; 

 and even in the Oriskany period, the beds are partly calcareous. The 

 West was therefore in certain parts still making limestones, while 

 the East interposed between its limestones extensive clay and sand 

 deposits. The limestones of the West prove that there were but 

 slight changes of level there during the long era when each stratum 

 was forming ; for, if great, they would have resulted in an extermi- 

 nation of the life, and a change, therefore, in the character of the 

 limestone. At the same time, the great thickness of the argillaceous 

 beds and sandstones of the East indicate great oscillations over the 

 Appalachian region ; during the Niagara period, they amounted, in 

 Pennsylvania, to at least 500 feet in the Oneida epoch, 1,500 feet in 

 the Medina, over 2,000 feet in the Clinton, 1,500 feet in the Niagara 

 and Salina, and 500 in the Lower Helderberg, — in all 6,000 feet. 

 In the Salina period, the subsiding area stretched up into New York, 

 west of its centre ; for it was there that the Salina beds were formed 

 to a thickness of 1,000 feet, with evidence in many parts of shallow- 

 water origin. 



After the Salina period closed, limestones (the Lower Helderberg) 

 were formed, some hundreds of feet thick, over the Hudson River 

 valley, and probably all the way to Montreal, showing that the sea 

 had again free access over eastern New York. East of the Green 

 Mountain region also, there was probably salt water and some lime- 

 stone making, along a large part of the Connecticut valley, and over 

 much of the country thence to the St. Lawrence Gulf. 



The conclusion cited from Mr. Billings, on page 250, that the 

 Trenton period, in the region of the Gulf of St. Lawrence, fails of 

 that commingling of European species which occurs in the period pre- 

 ceding and those following, accords with the fact that the Trenton 

 limestone was eminently continental ; it extending across the continent, 

 even over the Appalachian region ; and it sustains the conclusion 

 that the Trenton limestone was made in an interior sea, and hence 

 that, to the north, the outside barrier of that sea lay to the east of 

 the present coast line, and thus prevented the introduction of British 

 species. But in the following Cincinnati period there was a change 

 which resulted in making the Appalachian region again a region of 



