] 5 6 SKETC HES F CUE A TI X. 



" Mammoth Vein," at Wilkesbarre, Pennsylvania, is twen- 

 ty-nine feet thick. Add to the time occupied in the accu- 

 mulation of the coal the time which elapsed during the in- 

 undations, when shales, sandstones, and limestones accumu- 

 lated to an average of fifty times the thickness of the coal, 

 and we shall have at least double the above interval, oi 

 two hundred and eighty-eight thousand years, for the time 

 required to build up a series of coal-measures three thou- 

 sand feet thick. This is about the thickness of the Penn- 

 sylvania measures, while those of Nova Scotia are five 

 times as thick. 



It is not forgotten that allowance must be made for the 

 extraordinary luxuriance of Carboniferous vegetation, and 

 I offset this consideration by the fact that large quantities 

 of carbon, taken by it from the atmosphere, must have been 

 returned again by the partial decay and destruction of the 

 tissues, thus rendering them so difficult to detect in the 

 substance of the coal. Other calculations, based upon the 

 assumption that the coal-measures were accumulated at 

 the mouths of rivers, result in the determination of a 

 length of period equally enormous. But the whole history 

 of our world since the commencement of animal existence 

 is divided into over thirty periods, each corresponding to 

 that of the coal, and that portion of its history anterior to 

 the creation of animals was at least equally protracted ! 



The vegetation of this period was comparatively low in 

 rank. It was almost exclusively a flowerless vegetation. 

 But the sombre aspect of the prairie and forest comported 

 well with the absence of admiring intelligences, and the 

 low grade and character of the few beings which basked 

 in the sun or bathed in the waters of the Carboniferous 

 Age. The leading forms of vegetation were allied to 

 rushes, ferns, and club-mosses (see Fig. 65). Many of these 

 grew to colossal dimensions. Some of the rushes — Cala- 



