PALEOZOIC TIME — CAEBONIC. 709 



the Siibcarboniferous and the epoch of the Millstone grit ; and they were 

 continued even after the Carboniferous, during the Permian. 



These submergences, although quietly carried forward, played havoc with 

 the leaves, trunks, and stumps, floating them away for burial by the in- washed 

 sediments. Some of the transported stumps may occasionally have had 

 aboard large stones which they finally dropped, thus putting an occasional 

 " bowlder " into the forming beds. The encroaching waters at times flowed 

 with great force and plunging waves, as is shown not only by the formation 

 of coarse gravel beds (now conglomerates), but also by the erosion of the 

 rock deposits, and in some cases of the beds of vegetable debris. In Ver- 

 milion County, 111., as observed by F. H. Bradley, a portion of the Upper 

 Coal-measures, including shales, argillaceous limestones, and two coal-beds, 

 were carried away to a depth of 60 feet ; and, in the depression thus made, 

 a sandstone, which belongs at the top of the series, was laid down so as to 

 fill and overlie it. Also, on the same authority, in Vermilion County, Ind. 

 (adjoining the county just mentioned), the Millstone grit (here a pebbly 

 sandstone), under the Coal-measures, is cut off short and followed horizon- 

 tally by shale and limestone ; as if the grit stood as a bluff in the waters, in 

 which the latter rocks were deposited. Other evidences of erosion have been 

 described from these states, and also from Ohio, Kentucky, and Missouri. 

 The change of level over Iowa, Illinois, and Missouri, which permitted the 

 Coal-measures to spread northward beyond the limits of the Chester lime- 

 stone, the last of the Subcarboniferous beds, and even beyond the Kinder- 

 hook beds, was of the same nature with the oscillations above referred to. 

 No unconformity with the Subcarboniferous was produced except that of 

 overlap. The little value, as regards time divisions in geological history, of 

 unconformity by overlap or erosion is well illustrated by the facts here 

 stated. 



The coal-beds are thin compared with the associated rocks. But the time 

 of their accumulation, or the length of all the periods of verdure together, 

 may have far exceeded the time occupied in the accumulation of sands and 

 limestones. If there were but 100 feet of coal in all, it would correspond to 

 more than 500 feet in depth of vegetable debris. The sands and clays which 

 came in after each time of verdure put under heavy cover the thick bed of 

 vegetable debris which had accumulated, and thus the decomposition of the 

 plants and the change to coal took place, under the best of conditions for 

 coal-making. In some regions the coal-plants may have been drifted to 

 their places of deposit ; but this was not the usual way in North America. 



The great marsh from which proceeded the Pittsburg coal-bed of the 

 Upper Productive Measures, according to J. J. Stevenson, was the ''parent 

 marsh " also of the coal-beds above it in the series, times of temporary burial 

 being indicated by the intervening beds of shale and sandstone during the 

 progress of a very slow and intermittent subsidence. 



A coal-bed itself bears evidence of alternations of condition in its own 

 lamination, and even in the alternations in its shades of color. A layer one 



