504 CLIMATE OF COAL PERIOD. [Ch. XXV. 



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

 vailed 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 be 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 without any superincumbent coal, we may pre- 

 sume that frequently layers of vegetable matter were removed by 

 floods ; and in other cases, where the stigmaria-clays are for a certain 

 space covered with coal, and then prolonged without any such cover- 

 ing, the inference of partial denudation is still more obvious. 



In the Forest of Dean, in Gloucestershire, 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 undis- 

 turbed 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, especially when we remember how 

 often the roots of trees (Stiff 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 botanist taught that a 

 tropical climate was implied by the carboniferous flora, geologists 

 might well be at a loss to reconcile the preservation of so much vege- 

 table matter with a high temperature ; for heat hastens the decompo- 

 sition 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 resem- 

 bling that now enjoyed in the equatorial zone. Tree-ferns range as 

 far south as the southern part of New Zealand, and Araucarian pines 

 occur in Norfolk Island and Chili. A great predominance 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 corals, and cephalopoda of the Moun- 

 tain Limestone, — they belong to families of whose climatal habits Ave 

 know nothing ; and even if they should be thought to imply that a 

 warm temperature characterized the northern seas in the carboniferous 



