456 PENNSYLVANIAN PERIOD 



peat bogs escapes as gas (CO2, CH 4 , etc.), while the peat is being 

 changed to coal. If this is true, the seven-inch layer would be re- 

 duced to less than one and one-half inches, and a layer one foot in 

 thickness would require between 8,000 and 9,000 years. The aggre- 

 gate thickness of coal is as much as 100 feet in many places, and as 

 much as 250 feet in some. At the above rate of accumulation, 

 periods ranging from nearly 1,000,000 to nearly 2,500,000 years 

 would be needed for such thicknesses. It should be borne in mind, 

 however, that much depends on the rate of growth of Carboniferous 

 vegetation, which is not known. 



On the other hand, these figures refer to the coal only, not to 

 the Coal Measures. The greater part of the Coal Measures is shale 

 and sandstone, and of these formations there are thousands of feet, 

 even where the sediments were fine and their accumulation prob- 

 ably slow. It would hardly seem unreasonable to conjecture that 

 their deposition may have consumed as much time as that of the 

 coal. Doubling the above figures, we get 2,000,000 and 5,000,000 

 years respectively, figures which must be taken to mean nothing 

 more than that the best data now at hand indicate that the Penn- 

 sylvanian period was very long. 



Close of the period. After the long period of oscillation above 

 and below the critical level recorded by the Coal Measures, the 

 interior east of the Mississippi was brought above the level of the 

 sea, not to sink beneath it again during the Paleozoic era, and some 

 of it at no later time. This emergence marks the close of the 

 Carboniferous, and the inauguration of the Permian period. It is 

 also probable that the deformative movements which were to 

 develop the Appalachian Mountains began at this time. There 

 were notable changes also in the western half of the continent, for 

 the Permian system is much less widespread than the Pennsyl- 

 vanian. Where the Permian occurs, its constitution and fossils 

 indicate not only different relations of land and water, but different 

 conditions of erosion. 



Foreign Countries 



Europe. As in America, the oldest formation of the Upper 

 Carboniferous in Europe is in many places a conglomerate and sand- 

 stone formation, called the Millstone grit, in England. The Coal 

 Measures consist principally of shales, with sandstone and limestone. 

 Associated with these commoner sorts of rock, there are beds of coal 



