510 GEOLOGY. 



of change which is here held to separate periods (not epochs). As 

 already indicated, the geographic changes extended far beyond the 

 eastern interior, and even beyond the Mississippi basin. In Colorado x 

 there is a great break in the Carboniferous system which perhaps 

 corresponds to that just noted. Great thicknesses of the Mississippian 

 system were probably removed before the deposition of the Pennsyl- 

 vanian formations. The relations of the strata below and above the 

 unconformity are such as to show not only that great changes of level, 

 but probable orogenic movements supervened. Similar changes are 

 suggested by the relations of the earlier and later Carboniferous beds in 

 Utah, 2 Montana, 3 and in British Columbia, 4 though their contem- 

 poraneity with those of the Mississippi basin has not been established. 

 Even where there is no unconformity between the Lower Carboniferous 

 and the Coal Measures, as in most of the Appalachian belt, there was 

 a notable change in sedimentation, indicating physical changes of 

 consequence. Nowhere else in the whole course of the Paleozoic era 

 are so great physical changes embraced within the limits of one period. 

 On this ground, the Mississippian (or Lower Carboniferous) might 

 well be separated from the Pennsylvanian (Coal Measures) as a sepa- 

 rate system. If the Mississippian and Pennsylvanian are still regarded 

 as subdivisions of the Carboniferous period, it should be distinctly 

 understood that these epochs, and their corresponding series of for- 

 mations, are much more distinct, physically, than the epochs and series 

 of earlier Paleozoic periods. 



Thickness of the Mississippian System. 



In keeping with the variations in the sediments, the thickness of 

 the Mississippian system is exceedingly variable. In Pennsylvania, 

 where the group reaches its maximum development, there is a thick- 

 ness of 1400 feet of sandstone (Pocono), with 3000 feet of shale (Mauch 

 Chunk) above it; but so rapidly do the formations thin to the westward 

 that in the western part of the state the equivalent formations have 



1 Emmons, Orogenic Movements in the Rocky Mts., Bull. Geol. Soc. Am., Vol. I; 

 Mono. XXVII, U. S. Geol. Surv.; Anthracite-Crested Butte; and Ten Mile, Colo., 

 folios, U. S. Geol. Surv. Also Girty, Professional Paper No. 16, U. S. Geol. Surv. 



2 See 1 above. 



3 Montana folios, U. S. Geol. Surv. 



4 Dawson, Science, March 15, 1901. 



