544 GEOLOGY. 



spite of the fact that they were periodically depressed and degraded, 

 and so temporarily diminished in area. The result of the fluctuations 

 of land and water appears to have been in the long run to the advantage 

 of the land. If the apparent changes of level of the land were 

 really due to its rising and sinking, the sum of the risings exceeded 

 the sum of the sinkings. If, on the other hand, the apparent 

 changes of level of the land were due to fluctuation of the level of the 

 water, the average movement of its surface was downward. Before 

 the Pennsylvanian period, 1 the Ohio island had made its appearance, 

 arid-constituted a new sub-center of growth. ..-j With these several 

 land areas gradually (on the average) enlarging, it is clear that the 

 continuation of the process must ultimately have resulted in their 

 union. Barring the Coastal Plain, that time came, so far as eastern 

 North America is concerned, with the close of the Pennsylvanian 

 period. While therefore the Pennsylvanian system of rocks really 

 stands in much the same relation to the Mississippian system that 

 each of the preceding systems does to its predecessor, its surface dis- 

 tribution seems to depart from the usual rule. 



Some of the separate areas of the system shown in Fig. 240 have 

 probably been isolated since their deposition. Thus the eastern 

 interior coal-field (Illinois-Indiana area, Fig. 241) may have been 

 continuous with the western interior field (Iowa-Missouri-Arkansas 

 area), the separation having been effected by erosion along the 

 Mississippi, where the Coal Measures have been removed and the 

 Mississippian formations below exposed. These two coal-fields are 

 separated by a broad low anticline, 2 the existence of which allowed 

 the Mississippi River to reach the Mississippian series the more readily 

 in its down-cutting. The Coal Measures of different parts of Penn- 

 sylvania (Fig. 240), where the considerable altitude has favored ero- 

 sion, have been isolated, though probably once continuous. The figure 

 gives some idea of the extent of erosion in post-Carboniferous time. 

 The Coal Measures of Pennsylvania may once have been continuous 

 with those of the eastern Interior, via Kentucky and Tennessee, though 

 this is uncertain. The Michigan 3 and Rhode Island 4 areas (the latter 



1 Foerste, Bull. Geol. Soc. Am., Vol. XII, pp. 398-444. 



2 Hayes, 22d Ann. Rept. U. S. Geol. Surv., Pt. Ill, p. 16. 



3 Lane, ibid., pp. 307-331, and Repts. of the Michigan Geol. Surv. 

 'Shaler, Woodworth, Foerste, Mono. XXXIII, U. S. Geol. Surv. 



