EVOLUTIONAL GEOLOGl. 313 



In respect of coal there i.s much to sugg-est that its growth was rapid. 

 The Carboniferous period well deserves its name, for never before, 

 never since, have carbonaceous deposits accumulated to such a remark- 

 able thickness or over such wide areas of the earth's surface. The 

 explanation is doubtless partly to be, found in favorable climatal con- 

 ditions, but also, 1 think, in the youthful energy of a new and over- 

 mastering type of vegetation, which then for the first time acquired 

 the dominion of the land. If we turn to our modern peat bogs, the 

 only carbonaceous growths available for comparison, we find from data 

 given by Sir A. Geikie that a fairly average rate of increase is 6 feet 

 in a century, which might perhaps correspond to 1 foot of coal in the 

 same period. 



The rate of deposition has been taken as uniform through the whole 

 period of time recorded by stratified rocks; but lest it should be sup- 

 posed that this involves a tacit admission of uniformity, I hasten to 

 explain that in this matter wo have no choice. We may feel convinced 

 that the rate has varied from time to time, but in what direction or to 

 what extent it is impossible to conjecture. That the sun was once much 

 hotter is probable, but equall}^ so that at an earlier period it was much 

 colder; and even if in its youth all the activities of our planet were 

 enhanced this fact might not affect the maximum thickness of deposits. 

 An increase in the radiation of the sun, w^hile it would stimulate all the 

 powers of subaerial denudation, would also produce stronger winds and 

 marine currents. Stronger currents would also result from the greater 

 magnitude and frequency of the tides, and thus while larger quantities 

 of sediment might be delivered into the sea they would be distributed 

 over wider areas, and the difference between the maximum and average 

 thickness of deposits would consequently ])e diminished. Indications 

 of such a wider distribution may perhaps be recognized in the Paleozoic 

 systems. Thus we are compelled to treat our rate of deposition as 

 uniform, notwithstanding the serious error this may involve. 



The reasonableness of our estimate will perhaps best appear from a 

 few applications. Fig. 2 is a chart, based on a map by De Lappareut, 

 representing the distribution of land and sea over the European area 

 during the Cambrian period. The strata of this system attain their 

 maximum thickness of 12,000 feet in Merionethshire, Wales. They 

 rapidly thin out northward, and are absent in Anglesey; scarcely less 

 rapidly toward Shropshire, where they are 3,000 feet thick; still a 

 little iess rapidly toward the Malverns, where they are only 800 feet 

 thick, and most slowly toward St. Davids Head, where they are 7,400 

 feet thick. The Cambrian rocks of Wales were in all probability the 

 deposits of a river system which drained some vanished land once situ- 

 ated to the west. How great was the extent of this land none can say- 

 Some geologists imagine it to have obliterated the whole or greater 

 part of the North Atlantic Ocean. For my part I am content with a 



