37 



ORDOVICIAN PERIOD 



" 



II 



So oo 





o 5 

 T) > 



y U 

 OQ 



* 8 



'"JEi 

 o c/5 



West. of the Great Plains, the 

 system is present generally, 

 and the sections are somewhat 

 simpler than in the interior 

 or the east, limestone being a 

 conspicuous part of the system 

 here. 



General conditions in the 

 eastern part of the continent. 

 At no previous epoch was there 

 anything like such widespread 

 deposition of limestone within 

 the limits of our continent, as 

 in mid-Ordovician time, when 

 limestone was forming from 

 New England on the east, to 

 Georgian Bay on the north- 

 west, to Oklahoma and Texas 

 on the southwest, and Alabama 

 on the south, as well as in much 

 of the west. It is perhaps 

 equally worthy of note that in 

 the later part of the period, 

 mud (now shale) was deposited 

 over an almost equally exten- 

 sive area. This may mean that 

 the lands were so elevated as 

 to allow the streams to carry 

 more sediment to the sea, or 

 that conditions favored the 

 transportation of mud farther 

 from shore than formerly, or 

 both. All the Ordovician for- 

 mations of the interior and 

 the east bear within them- 

 selves evidence of shallow water 

 origin. 



Igneous rocks of Ordovician 

 age attain little importance in 

 North America. Their general 



