THE SILURIAN PERIOD 107 



New Jersey and Pennsylvania. The Delaware Water Gap is cut 

 through this formation. 



Overlying the Salina beds, but considerably more extensive, 

 are the limestones and waterlimes of Cobleskill, Rondout, and 

 Manlius ages which reach from Pennsylvania and New York west- 

 ward to Indiana and Wisconsin. 



Silurian of the West. — Definite subdivisions and correlations 

 of the Silurian strata of the West have not yet been made, but in 

 certain regions, like the Great Basin, there appears to be a prac- 

 tically unbroken succession of largely limestone strata ranging in 

 age from Middle Ordovician (Trenton) to Devonian. 



Thickness of the Silurian. — From central to western New 

 York the thickness of the Silurian system is from 1000 to 1500 

 feet. Its maximum thickness is from 2000 to 4000 feet in the 

 Appalachians, while in the Mississippi Valley the thickness is 

 generally less than 1000 feet. The Niagara limestone is a notable 

 exception to the usually greater thickness of the early Paleozoic 

 strata in the Appalachian region, since in Wisconsin it is some 700 

 or 800 feet thick, while in the east it is only from 100 to 300 feet. 



Igneous Rocks. — In North America the only igneous rocks 

 regarded as of Silurian age are some in Maine, Nova Scotia, and 

 New Brunswick. Igneous intrusives of later date have sometimes 

 penetrated Silurian strata. 



Physical History 



Early and Middle Silurian. — We have learned that, as a 

 result of physical disturbance toward the close of the Ordovician, 

 much of the interior Paleozoic sea was drained, causing the land 

 area to be so much enlarged as to have been more extensive than 

 at any time since the beginning of the Paleozoic era. This was 

 essentially the geographic condition of the continent at the begin- 

 ning of the Silurian. The boldest topographic feature was the 

 presence of the newly formed Taconic Range along the Atlantic 

 sea-board. Doubtless there were some areas of sedimentation, 

 but our present knowledge of the earliest Silurian physiography 

 of North America does not admit of their delimitation. 



This condition of the continent was not of (geologically) long 

 duration, because pretty early in the Silurian another great trans- 

 gression of the sea took place, gradually increasing the extent of 



