410 THE CARBONIFEROUS PERIOD 



any other Palaeozoic system, but they cannot yet be compared in 

 detail with those of the East. In the central portion of the In- 

 terior Sea there was likewise a deepening and extension of the 

 waters, forming clear seas, in which marine organisms nourished 

 luxuriantly, and great bodies of limestone were formed over areas 

 where the Devonian is very thin or altogether absent. The north- 

 ern islands of the Cincinnati uplift became joined to the northern 

 land, forming a long, narrow peninsula, while a strip was added 

 along the western side of the Appalachian land. The north- 

 eastern part of the Interior Sea was thus divided into two bays, 

 the larger one covering nearly all of Pennsylvania, the eastern 

 part of Ohio and Kentucky, and West Virginia, the other occupy- 

 ing the southern peninsula of Michigan and communicating with 

 the first by a narrow strait. So long as marine conditions lasted 

 in these bays, the rocks laid down in them were nearly all frag- 

 mental, conglomerates, sandstones, and shales (though with occa- 

 sional layers of limestone), and it is very difficult to correlate them 

 with the thick masses of limestone accumulated in the clearer and 

 deeper waters farther west. 



The Carboniferous strata are divisible into two great series, the 

 Upper Carboniferous or coal measures above and the Lower Car- 

 boniferous (often called Subcarboniferous) below, though it must 

 not be supposed that the formation of coal began simultaneously 

 in all the areas, for, as a matter of fact, we know that it did not. 

 In the Acadian province (Nova Scotia and New Brunswick) the 

 lower part of the Carboniferous system is made up of thick masses 

 of sandstone and conglomerate with overlying limestones, the 

 latter with inclusions of gypsum, which indicate the occasional 

 formation of closed lagoons. The thickness of these beds to- 

 gether amounts to 6000 feet. 



In Pennsylvania the lower members of the system are the 

 Pocono sandstone and the Mauch Chunk shales, which together 

 have a maximum thickness of 4000 feet. The hard Pocono 

 sandstone forms the summit of the plateau of that name and 

 of many of the Allegheny ridges. Westward and southward 

 the sandstone and shales thin away quickly, showing that their 



