420 



HISTORICAL GEOLOGY 



zoans, yet the lime was ultimately derived from the land by 

 solution. 



In the Appalachian trough (Figs. 392, 393), which was first formed 

 in the Cambrian, sands, muds, and limestones were laid down. The 

 Appalachian trough was separated from the Atlantic by the great 



Fig. 390. — A later stage of the Ordovician, when the land was greatly extended. 

 (Modified after Schuchert.) 



island or continent of Appalachia, whose eastern extent was unknown. 

 Its western border was a shifting shore which during certain times was 

 west of the present Mississippi River. Sandstones occur in the West, 

 where islands formerly existed, but are seldom extensive. In New- 

 foundland, Ottawa, and west of the Adirondacks, limestone is the 

 prevalent rock of the system, although shales are abundant, showing 

 either that the land was low or covered by vegetation which prevented 

 rapid erosion, or that the drainage of the land was not discharged in 

 the direction of these regions. 



