THE ORDOVICIAN PERIOD. 



331 



while in the interior it is to be measured by hundreds instead (see sec- 

 tions pp. 318-320). In Wisconsin and Iowa, where the system seems to 



Fig. 147. — Diagram illustrating one cause of the failure of Ordovician beds to appear 

 about a center of older rock. 



be full, that is, where sedimentation seems to have been continuous 

 from the beginning of the period to its end, the aggregate thickness 

 is somewhat less than 1000 feet. 



Width and position of outcrops. — The relation between the thick- 

 ness of a rock system and the width of the belt where it appears at 



CDSO € OSDC 

 H -I H H hhhh 



Fig. 148. Fig. 149. 



Fig. 148.— Diagram illustrating the double outcrop which the Ordovician, 0, should 



show in a truncated anticline where erosion has cut down to older formations. 



The two outcrops should appear on opposite sides of the axis of the anticline. 

 Fig. 149. — Ground plan illustrating the structure shown in the last figure. The 



Cambrian system, £7, is flanked on either side by the Ordovician system, 0, and 



this in turn by the Silurian, S, Devonian, D, and Carboniferous, C. The figure 



also shows the direction of the dip of the beds. 



the surface has already been set forth in connection with the Cam- 

 brian. The same principles are applicable here. In the interior, 

 where the system is relatively thin, it sometimes appears at the sur- 

 face in relatively wide belts or areas (Fig. 129), while in the eastern 

 mountains, where it is thick, it appears at the surface in a succession 

 of narrow and parallel belts. 



