THE ORDOVICIAN PERIOD 347 



which are used to mark the close of the latter period. It is 

 believed that, during ages of tranquillity of the earth's sur- 

 face, the forces which at times produce warping and moun- 

 tain folding accumulate power until finally the resisting 

 strength of the rocks is overcome, and the outer layers are 

 wrinkled and broken. This wrinkling is usually confined to 

 a small belt or district, but within that area the folding 

 and crushing may be intense. In the present instance the 

 first premonition of a change is afforded by the fact that 

 the clear seas of the Middle Ordovician in eastern United 

 States later became turbid with mud, so that the last 

 strata of the system are shales overlying the limestones. 

 Evidently changes in the activities of rivers or currents, or 

 both, were in progress, although it is not easy to prove just 

 what the changes were. In eastern New York the early 

 Silurian strata are found lying unconformably upon highly 

 folded rocks which are known to be of Ordovician age. 

 From this it is known that the recently deposited Ordovician 

 and older strata, in that region and somewhat farther south- 

 ward, were intensely deformed ; and also that the same region 

 became land and was subject to long-continued erosion. 

 The wide extent of the unconformity shows that much of 

 the eastern interior of the United States emerged at the same 

 time. 



During the compression of the rocks in the East, shales 

 became schists, and fossil-bearing limestone was altered to 

 marble in which nearly all trace of fossils has disappeared. 

 The local nature of this disturbance becomes evident when it 

 is found that in the adjacent regions of New York and New 

 Jersey the Ordovician rocks were only slightly disturbed at 

 this time, while in some portions of the Mississippi Basin they 

 did not even emerge from the sea. The obvious result of the 

 folding must have been a belt of mountains, perhaps of notable 

 height. Although these have since been totally cut away by 

 the erosive agencies, their site is occupied by the newer 

 Taconic Mountains of to-day, and so this disturbance which 



