THE ORDOVICIAN PERIOD 



421 



The Ordovician was a period of quiet, during which the epicon- 

 tinental seas gradually increased until the middle of the period (Fig. 

 389), at which time a larger portion of the continent was under water 

 than at any stage since the Pre-Cambrian, more than half of the con- 

 tinent being at this time submerged, the epicontinental seas, broken 



Fig. 391. — A stage later than Fig. 390, when the continent was again greatly 

 submerged. (Modified after Schuchert.) 



by peninsulas and large and small islands, extending at certain times 

 from ocean to ocean. The seas were for the most part much less ex- 

 tensive in the latter part of the period (Fig. 390), at which time de- 

 posits of mud were laid down over extensive areas. A submergence 

 almost equal to that earlier in the period (Fig. 389), however, again 

 occurred (Fig. 391), and epicontinental seas spread widely over the 

 continent. At the close of the period these seas were again drained, 

 and the outlines of the continent were probably not unlike those of 

 to-day. 



