Ch. XXV.] CLIMATE OF COAL PEEIOD. 395 



present period, thermal waters and hot vapors burst out from the earth 

 during earthquakes, and these would not fail to promote the disengage- 

 ment of volatile matter from the carboniferous rocks. 



Continuity of seams of coal. — As single seams of coal are continuous 

 over very wide areas, it has been asked, how forests could have prevailed 

 uninterruptedly over such wide spaces. In reply, it may be said that 

 swamp-forests in one delta may extend for 25, 50, or 100 miles, while in 

 a contiguous delta, as on the borders of the Gulf of Mexico, another of 

 precisely the same character may t-e growing ; and these may in after 

 ages appear to geologists to have been continuous, although in fact they 

 were simply contemporaneous. Denudation may easily be imagined in 

 such cases as the cause of interruptions, which were, in fact, original. 

 But as in all the American coal-fields there are numerous root-beds with- 

 out any superincumbent coal, we may presume that frequently layers of 

 vegetable matter wei'e removed by floods ; and in other cases, where the 

 stigmaria-clays are for a certain space covered with coal, and then pro- 

 longed without any such covering, the inference of partial denudation is 

 still more obvious. 



In the Forest of Dean, ancient river-channels are found, which pass 

 through beds of coal, and in which rounded pebbles of coal occur. 

 They are of older date than the overlying and undisturbed coal-measures. 

 The late Mr. Buddie, who described them to me, told me he had seen 

 similar phenomena in the Newcastle coal-field. Nevertheless, instances of 

 these channels are much more rare than we might have anticipated, espe- 

 pecially when we remember how often the roots of trees [Stir/mar ice) 

 have been torn up, and drifted in broken fragments into the grits and 

 sandstones. The prevalence of a downward movement is, no doubt, the 

 principal cause which has saved so many extensive seams of coal from 

 destruction by fluviatile action. 



Climate of Coal Period. — So long as the bonanist taught that a tropi- 

 cal climate was imphed by the carboniferous flora, geologists might well 

 be at a loss to reconcile the preservation of so much vegetable matter with 

 a high temperature ; for heat hastens the decomposition of fallen leaves 

 and trunks of trees, whether in the atmosphere or in water. It is well 

 known that peat, so abundant in the bogs of high latitudes, ceases to grow 

 in the swamps of warmer regions. It seems, however, to have become 

 a more and more received opinion, that-^ the coal-plants do not, on 

 the whole, indicate a climate resembling that now enjoyed .in the equa- 

 torial zone. Tree-ferns range as hv south as the southern part of New 

 Zealand, and Ai-aucarian pines occur in Norfolk Island. A great pre- 

 dominance of ferns and lycopodiums indicates moisture, equability of 

 temperature, and freedom from frost, rather than intense heat ; and we 

 know too little of the sigillarise, calamites, asterophyllites, and other 

 peculiar forms of the carboniferous period, to be able to speculate with 

 confidence on the kind of climate they may have required. 



The same may be said of the coi'als and cephalopoda of the Moun- 

 tain Limestone, — they belong to families of whose cHmatal habits we know 



