364 PALEOZOIC TIME — CARBONIFEROUS AGE. 



in trees, made for those very conditions, but, as through the exist- 

 ing tropics, under a moist climate, in the great denseness of the 

 jungles and forests, many plants starting up where but one would 

 have nourished under less favorable circumstances. Our peat 

 swamps are often referred to as a measure for the growth of plants 

 in the Coal era. But this is an assumption not based on a due 

 consideration of the facts. The peat plants of the present day are 

 species of the temperate zone alone, and are too different in kind 

 to warrant a comparison. 



3. General Geography of North America. — The Subcarbonife- 

 rous period was a time mainly of submerged continents ; the Car- 

 boniferous, of general emergence. The conglomerate (Millstone grit), 

 with whose formation the Coal period began, marks the transition 

 from the marine to the land period. 



(1.) Epoch of the Millstone grit. — The areas overgrown by Crinoids 

 became in the Millstone epoch covered to a great extent by pebbles 

 and sand. These coarse beds indicate strong currents or heavy 

 breakers ; and such would sweep the surface during an epoch of 

 slow emergence. The great thickness and coarseness of the beds 

 through Pennsylvania, along the Appalachian region, point out 

 that this was the border reef of the continent and the region 

 of great subsidences. The more sandy character of the beds of 

 this border in Virginia harmonizes with the general fact in earlier 

 time ; and so also do the little thickness and finer character of the 

 beds of Ohio and eastern Kentucky, — a region on the inner margin 

 only of the subsiding Appalachian area, not participating in the 

 great change of level. 



The coal beds, in this epoch of the Millstone grit, also show that 

 the continent was in this semi-emerged condition ; for every such 

 bed is proof that areas of land were here and there above the 

 ocean, where plants could grow. 



(2.) Epoch of the Coal Measures. — As the plants were land-plants, 

 and the beds cover a vast area stretching almost continuously from 

 the middle or eastern border of the Appalachian region to the far- 

 ther limits of Missouri and Kansas, this great continental region is 

 safely regarded as at times beyond the reach of the ocean. The 

 emergence, going on in the Millstone-grit epoch by slow steps of 

 progress, ended, therefore, in a great increase of the continental 

 lands. They not only extended from the remote Arctic down to 

 southern New York, but they spread west and south, — west beyond 

 Missouri, and south over Tennessee and part of Alabama. Farther 

 west, there were limestones of the Coal Measure epoch forming, 

 instead of coal ; and these indicate that the old interior sea still 



