396 THE DEVONIAN PERIOD 



Ontario and recurs in southern Illinois, while in the eastern region 

 it is found in east Canada and as a very thick mass in Maine. 



The Oriskany was followed by the grits, or fine conglomerates, 

 of the Schoharie stage, which have much the same distribution as 

 the former and are thickest along the line of the Appalachian 

 uplift. Next, a deepening of the water brought about the condi- 

 tions favourable for the formation of the great Corniferous lime- 

 stone, which has a much wider distribution than the preceding 

 stages and indicates a transgression of the sea over many areas 

 that had been low-lying lands. This limestone extends from the 

 Hudson River across New York to Michigan, and around the 

 islands of the Cincinnati uplift into Indiana, Illinois, Kentucky, 

 Missouri, and Iowa. It is largely made of corals, sometimes as 

 recognizable reefs, a famous example of which is at the Falls of 

 the Ohio, above Louisville. In the eastern province the Cornif- 

 erous is represented at Gaspe" (Quebec) by 2000 feet of sand- 

 stones and limestones, and by a coral limestone in northern 

 Vermont. 



A change of conditions in the northeastern bay of the Interior 

 Sea checked the formation of limestone, and on a slowly subsiding 

 bottom were laid down great masses of shales and shaly sandstones 

 (which constitute the Hamilton series), with a few feet of lime- 

 stone at the top in many places. The Hamilton has nearly the 

 same distribution as the Corniferous, but thins out much to the 

 west and south, and in the Mississippi valley is represented by 

 limestones. In the eastern province the Hamilton reappears at 

 Gaspe, where it is displayed as a very thick mass of sandstones, 

 and in Nova Scotia and New Brunswick as sandstones and shales. 



The Upper Devonian, or Chemung series, as formed in the north- 

 eastern bay, is an exceedingly thick mass of shales and shoal 

 water, ripple-marked sandstone, reaching in the Appalachian ridges 

 of Pennsylvania a thickness of 8000 feet, but thinning away to the 

 south and west. Indeed, over much of the Mississippi valley the 

 entire Devonian system is represented by a few feet of black 

 shale, a circumstance which it is not easy to explain. In the 

 Upper Devonian sandstone of Pennsylvania are great reservoirs of 



