728 REPOET— 1900. 



they ave not siifBcient even to suggest an average ; for this we must be content to 

 wait till stilHcient tidal observations have accumulated, and the disturbing effect 

 of the inconstancy of the sea-level is eliminated. 



It may be objected that in framing our estimate we have taken into account 

 mechanical sediments only, and ignored others of equal importance, such as lime- 

 stone and coal. With regard to limestone, its thickness in regions where systems 

 attain their maximum may be taken as negligible ; nor is the formation of lime- 

 stone necessarily a slow process. The successful experiments of Dr. Allan, 

 cited by Darwin, prove that reef-building corals may grow at the astonishing 

 rate of 6 feet in height per annum. 



In respect of coal there is much to suggest that its growth was rapid. The 

 carboniferous period, well deserves its name, for never before, never since, have 

 Carbonaceous deposits accumulated to such a remarkable thickness or over such 

 wide areas of the earth's surface. The explanation is doubtless partly to be found in 

 favourable climatal conditions, but also, I think, in the youthful energy of a new 

 and overmastering type of vegetation, which then for tlie first time acquired the 

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

 ceous 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 one 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 supposed that this involves 

 a tacit admission of uniformity, I hasten to explain that in this matter we 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 equally 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, while 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 be diminished. 

 Indications of such a wider distribution may perhaps be recognised in the Palaeozoic 

 systems. Thus we are compelled to treat our rate of deposition as uniform, not- 

 withstanding the serious error this may involve. 



The reasonableness of our estimate will perhaps best appear from a few appli- 

 cations. Fig. 2 is a chart, based on a map by De Lapparent, 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 northwards, and are absent in 

 Anglesey; scarcely less rapidly towards Shropshire, where they are 3,000 feet 

 thick ; still a little less rapidly towards the Malverns, where they are only 800 feet 

 thick ; and most slowly towards St. David's 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 situated to the we.st. 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 somewhat large island. What area of this island, we mav 

 ask, would suffice to supply the Cambrian sediments of Wales and Shropshire'? 

 Admitting that the area of denudation was ten times as large as the area of 

 deposition, its dimensions are indicated by the figure ah c d on the chart. This 

 evidently leaves room enough on the island to furnish all the other deposits 

 which are distributed along the western shores of the Cambrian Sea, while those 

 on the east are amply provided for by that portion of the European continent 

 which then stood above water. 



If one foot in a century be a quantity so small as to disappoint the imagina- 

 tion of its accustomed exercise, let us turn to the Caqibviau succegsion of Scan-. 



