Sn 



J L 



410 DEVONIAN PERIOD 



regions were contemporaneous, they were probably deposited in 

 waters which were not connected (Fig. 358). Toward the end of 

 the Hamilton epoch, the barrier which separated their waters seems 

 to have been removed sufficiently to allow the waters and the life 

 on opposite sides to mingle freely (Fig. 359). 



General Considerations 



Outcrops. While the Devonian system is widely distributed in 

 North America, it does not appear at the surface in large areas. 

 The reasons are substantially the same as those for the limited 

 exposures of earlier systems. The removal of Devonian from areas 

 it once covered is oddly shown near Chi- 

 cago, where a small remnant of Devonian 

 sediment has been found in a fissure in the 

 Fig. 360. Figure illus- Niagara limestone, as shown in Fig. 360. 



trating the occurrence of The limestone was apparently fissured 

 remnants of Devonian ma- , r ., ^ ,. , 



terial in fissures in Niagara before the Devonian sediments were de- 

 limestone, near Elmhurst posited upon it. Portions of the sedi- 

 (Cook Co.), Illinois. ments fell into an open fissure, carrying 



with them distinctive fossils (fish teeth). In this protected position, 

 the fossils escaped removal. 



Igneous rocks. Igneous rocks have little representation in most 

 parts of the system in North America, but in Nova Scotia, New 

 Brunswick, and Maine, and at some points in the west, there are 

 igneous rocks which appear to be of this age. In many places in 

 the west, Devonian strata have been affected by dikes and intru- 

 sions of later times. 



Close. The general quiet which had prevailed during the period 

 seems not to have ended at its close. Only in the eastern part of the 

 continent, so far as now known, in Maine, Nova Scotia, New Brun- 

 swick, and the adjacent region to the north were Devonian strata 

 notably disturbed at the close of the period. Elsewhere the for- 

 mations of the younger system rest on those of the older without 

 stratigraphic break. 



Economic Products 



The Upper Devonian is the chief source of oil and gas in western 

 Pennsylvania and southwestern New York, and is one of the sources 

 in West Virginia. The Middle Devonian is oil-producing in On- 

 tario. Within the regions of their occurrence, oil and gas are more 

 likely to be found under low anticlines than in other positions, 



