CARBONIFEROUS AGE. 359 



In the Interior Continental region, the submergence attending the 

 formation of these intervening rocks was mostly or wholly marine ; 

 for all the fossils thus far observed are those of marine species ; and 

 they occur in many strata of limestone, sandstone, and shale, through- 

 out the Coal-measures. In Central Pennsylvania, the evidences of ma- 

 rine life are uncertain. Over the great Mammoth bed of Wilkesbarre, 

 are shales (in the township of Hanover) containing bivalve shells; but 

 these may be of the fresh-water type of Unionidce. The thinner shales, 

 among the coal-beds of the Interior basin, and the limited arenaceous 

 layers may have been formed when the marshes became flooded with 

 fresh waters ; while the great sandstones and limestones and thicker 

 shales are all evidence that the former fresh-water marsh was followed, 

 through submergence, by a flood of marine waters. The extermina- 

 tion of the Lepidodendrids of the Lower Coal-measures was probably 

 connected with such a submergence. The marine waters probably 

 came in from the Interior basin to the southwest, and not from the 

 ocean on the east. 



The Lower Coal-measures extend to the most eastern limits of the 

 anthracite in Pennsylvania, and contain but little limestone, either in 

 the east or west. The Upper, above the Pittsburg bed, extend only 

 over the western portion of that State. This more western limit of 

 the Upper than the Lower section shows plainly that a rising of the 

 country had taken place more to the east, to a height that was too 

 dry for the marsh-vegetation of which coal was made. We observe, 

 further, that limestones occur in the Upper Coal-measures, and increase 

 much on going westward over the Interior basin ; and, finally, as has 

 been stated, they prevail extensively over the larger part of the Rocky 

 Mountain region. 



The coal-bed itself bears evidences of alternations of condition, in its own lamination, 

 or even in the alternations in its shades of color. A layer an eighth of an inch thick 

 corresponds to an inch, at least, of the accumulating vegetable remains; and hence the 

 regularity and delicacy of the structure are not surprising. Alternations are a conse- 

 quence of (1) the periodicity in the growth of plants and the shedding of leaves; (2; 

 the periodicity of the seasons, the alternations of the season of floods with the season of 

 low waters, or comparative dryness; (3) the occurrence, at intervals of several years, of 

 excessive floods. Floods may bring in more or less detritus, besides influencing the fall 

 and distribution of the vegetation. In some conditions, there would be a long steeping 

 of the vegetation in the waters, before it was put under the pressure of beds of clay or 

 sand; and the precise quality of the coal would be varied thereby, the decomposition 

 of the vegetation depending on the amount of water, the composition of that water, 

 and the length of time exposed. Newberry has suggested that bituminous coal has 

 taken the form of Cannel when the vegetation was reduced to a perfect pulp at the 

 time of the change to coal. 



The Coal period was a time of unceasing change, — eras of uni- 

 versal verdure alternating with others of wide-spread waters, destruc- 

 tive of all the vegetation and other terrestrial life, except that which 



