THE DEVONIAN PERIOD 



121 



Lower Devonian 



Oriskanian 

 series 



Esopus grit. 



Oriskany sandstone and Glenerie lime- 

 stone. 

 Connelly conglomerate. 

 Port Ewen limestone. 



IBecraft limestone. 

 New Scotland limestone. 

 Kalkberg limestone. 

 Coeymans limestone. 



For a long time the Helderbergian series was placed with the 

 Silurian system, but on the basis of careful study of its fossils, it is 

 now generally agreed that it really represents the lowest portion of 

 the Devonian system. This is a good example of the difficulty in 

 drawing the line between two systems when no sharp stratigraphic 

 break or unconformity exists. 



As stated in connection with the preceding system, so here the 

 reader should know that in many parts of America where definite 

 correlations have not been made, local subdivisions or stage names 

 are employed, and also that the lithologic character of the various 

 stages in New York may be quite different from those in other 

 regions. 



Distribution and Character of the Rocks 



General Distribution. — The map (Fig. 68) gives the surface 

 distribution of all known Devonian rocks in North America. A 

 comparison with the Silurian strata map (Fig. 59) will show that, 

 in the eastern part of the continent, these two rock systems are 

 very similar in distribution, though the Devonian is absent from 

 Newfoundland and is of much larger surface extent in New York. 

 As compared with the Silurian the only other important differ- 

 ences are the much larger Devonian area in the Mackenzie River 

 region and the much smaller areas in the Arctic Islands region. 



It should again be borne in mind that these surface areas of 

 Devonian rocks fall far short of indicating the actual former, or 

 even present extent of rocks of this age, because considerable 

 Devonian rock has been removed by erosion, and much is now 

 buried under later formations. Thus in the Appalachians and 

 the mountains of the western United States, the Devonian strata 

 have been highly folded with others, so that only the outcropping 



