122 HISTORICAL GEOLOGY 



edges are visible. In the Mississippi Basin, where the strata are 

 essentially horizontal, deep well borings have proved that the 

 Devonian strata are extensively distributed under cover of later 

 rocks. 



Lower Devonian Rocks. — The Helderbergian series is very 

 limited in its distribution, and is found almost wholly in eastern 

 North America in three regions: (1) Maine, eastern Quebec, 

 Nova Scotia, and New Brunswick; (2) the northern and middle 

 Appalachians; and (3) in the lower Mississippi Valley in Oklahoma, 

 southeastern Missouri, and western Tennessee and Kentucky. 

 Limestone almost everywhere makes up the series which ranges 

 in thickness up to 600 feet. In the West, this series has been rec- 

 ognized only in the great body of Paleozoic limestone in Nevada. 

 Rocks of this age are also known on Kennedy Island west of 

 northern Greenland. 



The Oriskanian series is chiefly represented by the Oriskany 

 sandstone, the other members of the series being only of mere 

 local importance. The Oriskany formation is extensively developed 

 from central New York southward through the Appalachian region 

 to Alabama, and in the eastern Mississippi Valley. Its thickness 

 varies from a few feet in New York, to several hundred feet in 

 western Maryland. In northern Maine, New Brunswick, and 

 Nova Scotia, the Oriskany (much of it limestone) is well developed 

 though not much studied. 



Middle Devonian Rocks. — The Ulsterian rocks, except the 

 Schoharie grit which is limited to eastern New York, are much 

 more extensive than the Lower Devonian. 



The Onondaga limestone formation extends from eastern New 

 York and Pennsylvania westward to northern Michigan and south- 

 ern Illinois, except over the Cincinnati anticline area. Its entire 

 absence from all but the northern portion of the Appalachians is 

 particularly noteworthy. Its thickness is seldom over 200 feet, and 

 it is often largely made up of Corals, as for example at the Ohio 

 River rapids near Louisville. In northern Maine, New Brunswick, 

 and Nova Scotia, the Onondaga limestone is widespread and appar- 

 ently many hundreds of feet thick. It also occurs at the south end 

 of Hudson Bay. In most of the Devonian areas of western North 

 America (see Fig. 68) , the Onondaga formation is doubtless present 

 though not yet carefully studied. 



The Erian series, represented by the Hamilton and Marcellus 



