THE SILURIAN PERIOD. 375 



feet. The material for that part of the formation which lies in northern 

 New York and Ontario probably came from the north. The lesser 

 thickness of the formation in this region, as compared with that of 

 Pennsylvania, suggests either that the rate of accumulation was slower, 

 or that the formation in Ontario does not represent the full series of 

 beds belonging to the epoch. 



The Ni ag aran Series. 



The Clinton formation. — The formation next younger than the 

 Medina is known as the Clinton formation. Where the younger forma- 

 tion overlies the older they are generally conformable, but the distri- 

 bution of the two is so different as to indicate that geographic changes 

 of importance were in progress as the Silurian period advanced. The 

 Clinton formation extends farther east and farther west than the Medina, 

 indicating an expansion of the eastern interior sea in both directions. 

 To the westward the Clinton extends to Lake Huron and Indiana, and 

 perhaps to northern Illinois and eastern Wisconsin, and even to north- 

 eastern Missouri. If it extends so far west, it is made up of beds which 

 have usually been classed as Niagara. It is not known whether the 

 Cincinnati arch was submerged at this time, or whether it was an island 

 in the Clinton sea. Southern Indiana and southern Illinois seem not 

 to have been submerged. The Clinton formation also occurs in the 

 Appalachian mountains as far south as Alabama and Georgia. It 

 seems probable from the evidence now in hand that the elongate basin 

 in which the sediments of the Clinton formation of this mountain belt 

 accumulated was separated from the interior sea, or at least not freely 

 connected with it. This inference is based chiefly on the dissimilarity 

 of the faunas of the beds classed as Clinton in the Appalachian belt and 

 in the interior. 



Sedimentary beds which are believed to belong to the Clinton epoch 

 have been recognized in Nova Scotia (a part of the Anticosti series) and 

 at a few other places northeast of the United States. In this region 

 marine sedimentation was probably continuous through the Ordovician 

 and Silurian periods, and the correlation of the strata with those of 

 the interior is attended with difficulty and uncertainty. The materials 

 for these beds, so far as clastic, were probably derived from land to the 

 west. 



