THE ORDOVICIAN PERIOD 



79 



since Lower Ordovician rarely occurs in any areas not also occupied 

 by the Middle Ordovician, the accompanying map shows the 

 surface distribution of practically all Ordovician strata. By com- 

 paring the maps (Figs. 26 and 41) it will be seen that the distri- 



Fig. 41 

 Map showing the surface distribution (areas of outcrops) of chiefly Middle 

 and Upper Ordovician strata in North America. (By W. J. M., based upon 

 maps by Bailey Willis, U. S. Geological Survey.) 



bution of Ordovician rocks is essentially the same as that of Upper 

 Cambrian, with two important differences. These differences are 

 the presence of two large areas of Ordovician west of Hudson Bay, 

 and a number of smaller areas in the Arctic Islands region. 



As in the case of the Cambrian, so the surface distribution of 



