AMERICAN DEVONIAN 397 



petroleum and natural gas. Along the eastern shore of the Che- 

 mung sea was accumulated an immensely thick sandstone, which 

 was formerly supposed to represent a distinct series and called the 

 Catskill; the maximum thickness of this sandstone (7500 feet) 

 occurs in eastern Pennsylvania. 



Eastern New York and Pennsylvania were, then, for long ages a 

 slowly sinking marginal sea-bottom, on which, as in a great trough, 

 were accumulated immense masses of shallow-water deposits, with 

 occasional limestones when an increased rate of subsidence deep- 

 ened the water. During Cambrian and Ordovician times similar 

 conditions had prevailed southward along the line of the future 

 Appalachian range, but in the Silurian and still more in the Devo- 

 nian, sedimentation became chiefly concentrated in the northern 

 half of the trough, the subsidence of the southern portion having 

 become exceedingly slow and intermittent. 



The course of deposition of sediments which occurred during 

 Devonian times in the western portion of the continent was so 

 entirely different from the succession of sedimentation in the east- 

 ern half, that it is very difficult to correlate the subdivisions in 

 the tw T o regions, whose seas may have been separated by an un- 

 broken land area. Far to the north, in the Mackenzie River 

 region of Canada and coming down into Manitoba, the Devonian 

 is represented by about 1000 feet of limestones and shales, which 

 appear to belong to the middle and upper part of the system. 

 Another strip of Devonian rocks follows the main range of the 

 Canadian Rocky Mountains, extending southward into Montana. 

 The Front Range in Colorado was apparently a land area, for 

 there the Carboniferous strata rest upon the Cambrian and Ordo- 

 vician, as is also true of central Texas and the Black Hills of South 

 Dakota ; but in southwestern Colorado the Devonian reappears. 

 In parts of the Grand Canon region thin patches of Devonian 

 strata are found lying upon the upper Cambrian sandstones. In 

 the Wasatch Mountains of eastern Utah 2400 feet of quartzites and 

 limestones belong to the Devonian. In Nevada, on the other 

 hand, was a comparatively deep and tranquil marine basin, in 

 which deposition would seem to have gone on uninterruptedly from 



