THE DEVONIAN PERIOD 359 



largely carried off and the Devonian sea would have en- 

 croached upon a plain underlain by still older rocks. 



North America at the beginning of the Devonian period. 

 Although this low-lying land seems to have stretched from 

 Michigan and Virginia westward over much of the present 

 Mississippi Basin, the sea had by no means entirely retreated 

 from the continent. In what is now the lower Great Lakes 

 region and again in Utah, Nevada, and Montana, the deposi- 

 tion of limestone and shale went on from the Silurian far into 

 the Devonian. Considering the isolation of these localities, it is 

 not surprising that the fossils in the one place bear little rela- 

 tion to those of the other. No more do the animals which 

 inhabit the seas off California and New England to-day. 



DEVONIAN IN THE WEST 



As the Devonian period progressed, the events in one region 

 were not necessarily the same as those in another. In Utah, 

 for example, lime ooze and mud were deposited uninterrupt- 

 edly throughout the period, with the result that limestone 

 about 1000 feet thick now represents the Devonian in that 

 region. So free from disturbing influences was this part 

 of western United States that the animals of the western 

 sea underwent only very slow changes. The fossils in the 

 youngest beds of the system do not seem to differ widely 

 from those in the oldest. The conditions elsewhere were in 

 contrast to this. 



DEVONIAN IN THE EAST 



Helderberg limestone. In eastern United States the 

 period was marked by the gradual reexpansion of the epicon- 

 tinental sea, attended by important changes in the relations 

 of the land and water bodies. At first all the eastern lands 

 seem to have been low, for if land masses had been eroded 

 rapidly, the derived sediments would surely have formed 

 clastic rocks in the adjacent seas. As it was, only limestone 



