376 PALAEOZOIC SYSTEM OF ROCKS. 



Carboniferous times, there was high land to the north, east, and west of 

 this field, and the black area, representing the Coal-measures, was then a 

 trough, into which, therefore, drained rivers from every side except the 

 south. This trough was sometimes a coal-swamp, sometimes a lake 

 emptying southward, sometimes an arm of the sea connecting with the 

 ocean southward. When it was a coal-marsh, a coal-seam was formed ; 

 when a lake, sands and clays were deposited by the rivers ; when an 

 arm of the sea, marine deposits — limestones — were formed. 



This alternation of conditions we explain as follows : There were 

 three forces at work on this area : 1. A general continental upheaval, 

 affecting this along with all other parts of the continent ; 2. An up- 

 building by sedimentary deposit. 3. A local subsidence. The evidence 

 of all these is complete. The continental upheaval, as we have already 

 seen, was unceasing throughout the previous periods, and, as we shall 

 see, continued throughout the subsequent periods. The up-building by 

 sediments and the pari passu subsidence are as clearly marked as in 

 deltas of the present day, by shore-marks, by shallow-water fossils, and 

 especially by forest-grounds repeated through several thousand feet of 

 vertical thickness. The existence of these three forces, therefore, is 

 not a doubtful hypothesis. Now, the first two would tend to reclaim, 

 the third to submerge, the area. When the reclaiming forces pre- 

 dominated, the area became swamp-land, and covered with coal vegeta- 

 tion, and the river- water, strained through the thick growth, slowly went 

 southward by a kind of seepage. When the submerging forces pre- 

 dominated, the area became a lake, and sediments in great quantities 

 were brought down by the rivers. It is possible, perhaps probable, that 

 correlative with the more rapid local subsidence which formed the lake 

 there was also a more rapid elevation of the high lands on all sides, pro- 

 ducing more torrential river-currents and greater sedimentary deposits. 

 Now and then, at long intervals, the subsidence would bring the area 

 below sea-level, and would thus form an interior sea, or mediterranean. 

 During such times, limestones would be formed, and marine animals 

 would be imbedded as fossils. 



b. Western Coal-Fields. — The Central and Western coal-fields may 

 be regarded as one, having been subsequently separated by denudation. 

 This immensely extensive field may have been, like the Appalachian, a 

 hollow surrounded on all sides by higher land. If so, the western land 

 has since been submerged, and covered by more recent deposits. Or it 

 may have been an extensive jungly flat, bordering a western sea, the 

 many small rivers with inosculating deltas, flowing westward and seep- 

 ing through the thick, marshy vegetation. There were here far less 

 mechanical sediments, because less high land, and far more marine 

 deposits, because there was a larger and opener sea ; but, in other re- 

 spects, the process may be regarded as similar. 



