368 PALAEOZOIC TIME— CARBONIFEROUS AGE. 



taining marine shells. The thinner shales among the coal beds and 

 limited arenaceous layers may, however, 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 extermination of the Lepido- 

 dendra of the Lower Coal measures was probably connected with 

 such a submergence. 



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, 

 reach east only over the western portion of that State. This more 

 western limit shows plainly a rising of the country 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 are common 

 in the Upper Coal measures, and they increase much going west- 

 ward ; 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 consequence 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 vege- 

 tation 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 decom- 

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

 of that water, and the length of time exposed. Newberry has suggested that bitu- 

 minous coal has taken the form of Cannel when the vegetation was reduced to 

 a perfect pulp at the time of the change to coal. 



Conclusion. — The Coal period was, then, a time of unceasing change, 

 — eras of universal verdure alternating with others of wide-spread 

 and destructive waters, destructive of all the vegetation and land- 

 life except that which covered regions beyond the Coal-measure 

 limits. According to the reading of the records, it was a time of 

 great forests and jungles, and of magnificent foliage, but of few or 

 inconspicuous flowers ; of Acrogens and Conifers, with no Angio- 

 sperms ; of marsh-loving insects, Myriapods and Scorpions as well as 

 Crustaceans and Worms, representatives of all the classes of Arti- 

 culates, but not the higher insects that live among flowers ; of the 



