CHAPTER VII. 



THE SILURIAN (UPPER SILURIAN) 1 PERIOD. 



Formations and Physical History. 



The physical changes which brought the Ordovician period to a 

 close marked also the inauguration of the Silurian. These changes, 

 it will be recalled, involved (1) movements which affected small areas 

 intensely (orogenic), and (2) movements which affected broad areas 

 slightly (epeirogenic) . From the standpoint of continental history, 

 the gentler movements were the more significant, since they affected 

 larger areas. It is not to be understood that these changes, whether 

 orogenic or epeirogenic, were sudden or violent. Rather is it to be 

 supposed that the formation of mountains and the emergence of large 

 parts of the North American continent from the epicontinental sea 

 which had covered it during the Ordovician period, occupied a con- 

 siderable interval of time, including the later part of the Ordovician 

 and the early part of the Silurian. After these changes had taken 

 place, the area of land shown in Fig. 129 was greatly enlarged, was 

 indeed more considerable than at any previous time since the early 

 Cambrian (see Fig. 90, p. 220). With the increase in the area of land 

 came lengthened streams, and presumably increased erosion, though 

 the land areas were probably not of sufficient extent to give rise to 

 streams equal in size to the largest of those of the present time. 



Could the relations of land and water at the beginning of the Silurian 

 be accurately defined, their statement would at the same time define 

 in a general way the areas where the earliest sedimentation took place. 

 Not only this, but it would also define in a general way the areas where 

 sedimentation was rapid and where slow, for then, as always, such 

 areas must have stood in more or less definite relation to the shores 

 of the oceans. It is safe to assume that at the opening of the Silurian 



1 When the name Lower Silurian is used instead of Ordovician, Upper Silurian 

 is used in place of Silurian, as that term is here used. 



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