AMERICAN 411 



great thickness at the northeast was due to the rapid deposi- 

 tion of sediment upon the sinking bottom of that part of the bay. 

 In Ohio the Waverly beds, representing the same time, are a 

 mass of shales 700 feet thick, with some sandstone and lime- 

 stone at the top. In the Michigan bay were laid down the 

 Marshall beds, sandstones, grits, and shales below, followed by 

 shales with some limestone and gypsum, which point to a tem- 

 porary closing of the straits and the conversion of the bay into 

 a salt lake. The barrier was, however, removed and the bay 

 deepened, allowing the formation of a marine limestone. 



"Westward from the bays the deepening of the open Interior 

 Sea was followed by the accumulation of the great masses of lime- 

 stone which constitute the Mississippian series, and which were 

 formed from a most luxuriant growth of corals, brachiopods, and 

 crinoids. Six different stages and substages (see table) may be 

 distinguished in these limestones, and evidences of shifting coast 

 lines are not wanting. Thus, the Kinderliook extends farther 

 north and west than the Osage, while the St. Loins represents 

 a renewed transgression of the sea northward. The Mississip- 

 pian limestones have a thickness of 1200 to 1500 feet in southern 

 Illinois, thinning out northward, where the coal measures overlap 

 them and rest upon the Devonian. These limestones extend over 

 southern Indiana, Illinois, Iowa, Missouri, Kentucky, Tennessee, 

 and southwestward to Arkansas and Texas. 



Over the eastern part of the Interior Sea, where there had been 

 but scanty deposition during Devonian times, a sinking sea-bottom 

 and deeper water in the Lower Carboniferous were favourable to 

 the formation of limestones. In West Virginia occur nearly 

 1300 feet of sandstone and shale, with 800 feet of limestone. 

 In Virginia are 2000 feet of limestones, sandstones, and shales, 

 with many thin seams of coal, some of them workable. The 

 formation of peat bogs thus commenced in this region, as also in 

 Nova Scotia, before it did in Pennsylvania, and to distinguish 

 them, these lower coal-bearing beds are often called the False 

 Coal Measures. 



At the close of Lower Carboniferous time occurred wide-spread 



