THE ORDOVICIAN PERIOD 



513 



Igneous rocks. Igneous rocks of Ordovician age attain little 

 importance in North America. Their general absence is in harmony 

 with the quiet which characterized the period. 



General Conditions and Relations of the Ordovician System 



Position of beds. As originally deposited, the Ordivician beds 

 probably dipped away from such lands as then existed. Thus on 

 the south side of the land of northern Wisconsin (Fig. 382), the Or- 

 dovician sediments must have dipped slightly to the south, and on 

 the east and west sides, to the east and west respectively. The 

 same relations held about every land area. Over great areas in 

 the interior, this original and simple plan of stratigraphy has been 

 ^ but little modified (Fig. 378). 



( \ \ ^. In other regions, deformation of 



the strata has completely changed 

 their original positions. Thus in 

 the Appalachian Mountains, where 

 the sediments were derived prin- 

 cipally from the land to the east, 

 and where the beds doubtless had 

 a slight dip to the west at the 

 time of deposition, they now dip 



Fig. 381. Section showing the j n various directions and at vari- 

 position and relations or the , 



Ordovician beds in the moun- ous angles, as the result of folding. 



Length of Faulting has complicated their 

 A miles. 



structure still further (Figs. 379 



and 380) . The strata are in simi- 



lar positions in some parts of Arkansas (Fig. 381), Oklahoma, 

 and the various mountain ranges of the west. 



Condition of the formations. The sediments have undergone 

 more or less alteration since their deposition. In some places the 

 changes have been slight, and in others great. The larger part of 

 the Ordovician sands are now in the condition of sandstone, the 

 larger part of the muds in the condition of shale, and most of the 

 limestone is still essentially non-met amorphic. But where dynamic 

 action has been great and where the original position of the strata has 



tains of Arkansas. 

 section, about \* 

 (Penrose, Ark. Geol. 



