354 PALEOZOIC TIME. 



Ferns of the tropics are most luxuriant in moist woods. The Lepi- 

 dodendrids, by their association with the Sigillarids and Ferns, show 

 that the same conditions (as is now the case with their kin, the Ly- 

 copodia) favored their development. In fact, Lycopods, Equiseta, 

 and most Ferns, are plants that like shady as well as moist places. 

 Adding, then, the prevalent moisture and warmth to the excess of 

 carbonic acid in the atmosphere, we should be warranted in conclud- 

 ing that, even if there were less sunshine than at the present time, 

 vegetable growth must have been more exuberant than it is now, es- 

 pecially in the colder temperate zones. This exuberance would not 

 have shown itself in thick rings of growth, in trees made for those 

 very conditions, but, as through the existing tropics under a moist cli- 

 mate, in the great denseness of the jungles and forests, many plants 

 starting up where but one would have flourished under less favorable 

 circumstances. Our peat swamps are often referred to, as a measure 

 for the growth of plants in the Coal era. But while they illustrate 

 well the mode of making beds of vegetable debris, their rate of prog- 

 ress may be no safe criterion as to the rate in Carboniferous swamps. 

 The peat- plants of the present day are species of the temperate and 

 colder zones, and are too different in kind to warrant a comparison. 



3. Geographical Conditions over North America, during the Prog- 

 ress of the Carboniferous Period. — The Subcarboniferous was a 

 period of submerged continental regions ; and the Carboniferous of as 

 extensive an emergence ; not continuous emergence, but prolonged 

 and repeated emergences with little change of level, alternating with 

 slight or partial subsidences. 



The conglomerate, called Millstone-grit, with whose formation the 

 Coal-period began, marks the transition from the marine to the terres- 

 trial period. The area that had been covered with fields of Crinoids 

 was swept during this epoch by currents and waves, which left the 

 surface under a great depth of pebbles and sand. The coarseness of 

 the beds along the Appalachian region, in Pennsylvania, points out 

 that this was the border -reef of the continent ; and the great thick- 

 ness of the deposits, — 1,100 feet, — that it was a region also of pro- 

 found, though slowly progressing, subsidence. The more sandy char- 

 acter of the beds of this border in Virginia harmonizes with the 

 general fact in earlier time ; and so also do the little thickness and 

 liner character of the beds of Ohio and eastern Kentucky, — a region 

 on the inner margin of the subsiding Appalachian era, not participating 

 so fully in the great change of level. 



The coal-beds 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, for a long time, above the ocean, where plants 

 could grow. 



