462 Notices of Metnoivs — Prof. Sollas's Address to Section C. 



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 limestone 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 limestone 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 six 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 favour- 

 able climatal conditions, but also, I think, in the youthful energy 

 of a new and overmastering 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 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 difi'erence between the maximum and average thick- 

 ness of deposits would consequently be diminished. Indications of 

 such a wider distribution may perhaps be recognized in the Palaeozoic 

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

 uniform, notwithstanding the serious error this may involve 



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

 the imagination of its accustomed exercise, let us turn to the 

 Cambrian succession of Scandinavia, where all the zones recognized 

 in the British series are represented by a column of sediment 290 

 feet in thickness. If 1,600,000 years be a correct estimate of the 

 duration of Cambrian time, then each foot of the Scandinavian 

 strata must have occupied 5,513 years in its formation. Are these 

 figures sufficiently inconceivable ? 



In the succeeding system, that of the Ordovician, the maximum 

 thickness is 17,000 feet. Its deposits are distributed over a wider 



