THE DEVONIAN PERIOD 125 



(and Frankfort), Salina, and (Marcellus to) Portage (respectively), 

 due to an increase of terrigenous material, and each was closed by 

 a more or less widespread emergence of the sea-bottom. Each 

 began with a subsidence which gradually extended to a maximum 

 at the time when the great limestone was formed. The parallel- 

 isms are not exact, but they are certainly suggestive." x 



Thickness of the Devonian. — In the northern Appalachian 

 Mountains the Devonian system attains a maximum thickness of 

 some 14,000 or 15,000 feet. In New York state the system has a 

 thickness of fully 4000 feet. Over much of the upper Mississippi 

 Valley the thickness is generally less than 1000 feet, though rather 

 locally, in Ohio, a thickness of fully 3000 feet is reached, 2600 feet 

 of this being Upper Devonian shales practically equivalent to the 

 Portage and Chemung beds of the east. In Nevada the system 

 appears to show 6000 feet of limestone and 2000 feet of shale. 



Igneous Rocks. — As in the earlier Paleozoic periods, evidences 

 of igneous activity in the Devonian period are almost lacking. 

 Some lava sheets in New Brunswick, Nova Scotia, eastern Quebec, 

 and northern California, appear to be interbedded with Devonian 

 shales. These occurrences prove at least some volcanic activity 

 during the period. 



Physical Histoky 



Early Devonian. — In earliest Devonian (Helderberg) time 

 most of North America appears to have been dry land. Inspection 

 of the Paleogeographic map (Fig. 69) of that time shows that 

 marine waters occupied only three rather limited areas in the east. 

 These were Nova Scotia-New Brunswick; the northern Appala- 

 chian region to central New York; and a portion of the southern 

 Mississippi Basin. Of these the first two were doubtless connected 

 with the Atlantic, and the last with the Gulf Basin as the map 

 suggests. These three submerged areas must have been pretty 

 freely connected, probably along the Atlantic Coast, because the 

 faunas are so similar. Since the Helderberg formation is chiefly 

 limestone, the waters were clear and this implies no adjacent high 

 lands, or at least no rapid erosion. In the west an arm of the sea 

 must have reached into the Nevada Basin as proved by the 

 Helderberg limestone there, the connection with the Pacific prob- 



1 W. B. Scott: An Introduction to Geology, 2nd Ed., pp. 577-578. 



