CHAPTER XVIII 

 THE SILURIAN (UPPER SILURIAN) 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 

 included (1) movements which affected small areas intensely, and 

 (2) movements which affected broad areas slightly. From the 

 standpoint of continental history, the latter were the more important. 

 These changes were doubtless accomplished slowly, and after they 

 had taken place, the area of land in North America was greater 

 than at any time since the early Cambrian. The increase in land 

 meant lengthened streams, and presumably increased erosion. 



Could the distribution of land and water at the beginning of 

 the Silurian be defined accurately, it would define also the areas 

 where the earliest marine sedimentation of the period took place, 

 and, in a general way, the areas where sedimentation was rapid and 

 where slow, for then, as always, the rate of sedimentation must 

 have stood in more or less definite relation to the shore-lines. It is 

 safe to assume that at the opening of the Silurian period beds of 

 clastic sediments were accumulating about the immediate borders 

 of the lands, and as far out as waves and currents were able to trans- 

 port abundant detritus, and that elsewhere sediments of organic 

 origin were relatively more important. Though sedimentation 

 was interrupted in the regions which emerged from the sea during 

 the transition from the Ordovician period to the Silurian, such 

 interruption was not universal, and the Silurian strata are locally 

 conformable on the Ordovician in the continent, and generally, it 

 is to be presumed, in the ocean basins. 





