THE PENNSYLVANIAN PERIOD 641 



demanded by the coal. Doubling the above figures, we get some- 

 thing like 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 Pennsylvanian 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 again beneath it during the Paleozoic era, and some 

 of it at no later time. This emergence marks at once 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 wide-spread than the Carboniferous. 

 Where the Permian occurs, its constitution and its fossils indicate 

 not only different relations of land and water, but different condi- 

 tions of erosion, and the absence of the sea from some areas where 

 deposition was in progress. 



IN FOREIGN COUNTRIES 



Europe 



As in America, the oldest formation of the Upper Carboniferous 

 in Europe is often a conglomerate and sandstone formation, the 

 Millstone grit, which in some parts of Britain attains a great thick- 

 ness. The Coal Measures of western Europe, like those of eastern 

 North America, consist principally of shales (and clays), with sub- 

 ordinate amounts of sandstone and limestone. Associated with 

 these commoner sorts of rock, there are beds of coal and clay-iron- 

 stone, both of which occupy positions corresponding, in all essential 

 respects, with those of similar formations in eastern North America. 

 There is workable coal in Great Britain, Ireland, Belgium, France, 

 Spain, Germany, Austria, and Russia, but the total area of pro- 

 ductive coal in Europe is much less than in America. The num- 

 ber of coal seams is large in many places. Thus in Westphalia the 

 number of workable beds is said to be 90. The aggregate (maxi- 

 mum) thickness of the coal in Lancashire is 150 feet, and in West- 



