584 GEOLOGY. 



Close of the Carboniferous Period. 1 



After the long period of oscillation above and below the critical 

 level recorded by the Coal Measures, the interior area east of the Missis- 

 sippi 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 uplift 

 marks at once the close of the Carboniferous, and the inauguration 

 of the Permian period. It is also probable that the deformative move- 

 ments which were to result in the folding and uplift of the Appalachian 

 mountains began at this time. Dikes of igneous rock affect the Car-, 

 boniferous strata of northwestern Kentucky and southern Illinois, 

 but the date of their intrusion has not been determined. It seems 

 not improbable that it was during the general period of deformation 

 which marked the close of the Paleozoic; but this is conjecture. 



There were also notable changes in the western half of the con- 

 tinent, for the strata of the Permian period are much less wide- 

 spread than those of the Carboniferous. Where they occur, their 

 constitution and their fossils are such as to indicate not only different 

 relations of land and water, but different conditions of erosion, and 

 the absence of the sea from some areas where deposition was in progress. 

 In the Sierras, 2 too, there seems to have been deformation and meta- 

 morphism of the Carboniferous beds at some time antedating the 

 Jurassic period. The exact date of the disturbance has not been 

 fixed, but it may have been contemporaneous with that in the Appa- 

 lachian Mountain region. 



In America, the Permian has usually been regarded as the closing 

 stage of the Carboniferous. This classification seems best to fit the 

 facts of the eastern part of the continent, but when the relations of 

 the Permian in other parts of the world are considered, it seems more 

 fitting to regard the Permian as a distinct period. 



In Foreign Countries. 



Europe. 



General. — As in America, the oldest formation of the Upper Car- 

 boniferous in Europe is often a conglomerate and sandstone formation, 

 the Millstone grit, which in some parts of Britain attains a thickness 



1 For general account of Appalachia during the Paleozoic Era see Willis, Md. 

 Geol. Surv., Vol. IV, pp. 23-88. 



2 See description of the Gold Belt, in the late folios of the U. S. Geol. Surv. 



