THE DEVONIAN PERIOD 



453 



Geography. — The close of the Silurian found few epicontinental 

 seas in North America. In the east (Fig. 425) the Appalachian 

 trough, portions of New York, and certain areas in the Maritime 

 Provinces of Canada were covered with seas, and a bay extended from 

 the Gulf of Mexico toward the north along the valley of the Missis- 

 sippi. In the west an arm of the sea extended from the Pacific across 

 the site of the Sierra 

 Nevada into Utah. 

 The outlines of these 

 seas were not con- 

 stant but changed 

 from stage to stage. 

 Later in the period 

 (Fig. 426) the seas 

 spread widely over 

 the continent, calling 

 to mind the sub- 

 mergent condition of 

 the Middle Ordo- 

 vician and Middle 

 Silurian. 



In New York state 

 the formations of the 

 first half of the De- 

 vonian are for the 

 most part limestones 

 with occasional shales 

 and sandstones, but 

 in the later half of 

 the period shales and 

 sandstones predomi- 

 nate. The shales and 

 sandstones were 



brought into the sea by streams from the Taconics of Massachusetts, 

 and probably from land areas which existed to the north in Canada. 

 The Devonian strata cover a greater area at the surface in New 

 York than any other rocks, and their combined thickness is more 

 than 4000 feet (Fig. 427). They are much thicker in Pennsylvania, 

 but thinner in the Mississippi Valley, and are said to be 8000 feet 

 thick in portions of Nevada. 



Fig. 425. — Map showing the probable outline of 

 North America during a portion of the Lower Devonian. 

 (Modified after Schuchert.) 



