286 Correspondence — Mr. James Croll. 



omissions in it. I may mention, however, that the main object of 

 the paper was not so much to determine the thickness of the sedi- 

 mentary rocks as to direct attention to a method how this might be 

 done. On this subject all hitherto appears to have been little else 

 than mere conjecture. My object was to endeavour to bring the 

 matter out of the regions of mere opinion into that of positive 

 knowledge. But even assuming Mr. Poulett Scrope's conclusions 

 to be perfectly correct, viz., that if we take into account the various 

 sources of sedimentary accumulation at the bottom of the ocean, 

 omitted by me, it will double the figures in my estimate, and give 

 5,000 feet instead of 2,500 feet as the mean thickness of the sedi- 

 mentary rocks, still, this is a very low figure. To know with 

 tolerable certainty that the mean thickness of the sedimentary rocks 

 lies somewhere between 2,000 and 5,000 feet is surely a considerable 

 advance on our previous knowledge in this direction. 



But is it probable that the amount of materials supplied by the 

 three agencies referred to by Mr. Poulett Serope would equal that 

 supplied by sub-aerial denudation ? 



Take the first, viz.. Marine denudation. Suppose the mean height 

 of the coast line of the globe, now being cut down by the action of 

 the sea, to be 25 feet, and the mean rate at which the sea is ad- 

 vancing on the land to be one foot in a century ; the amount of 

 denudation thus effected would amount to only -rrW ^^'^ ^^ ^^' 

 aerial denudation.^ The amount of material supplied by marine 

 denudation would therefore add only 1^ feet to the thickness of the 

 sedimentary rocks. But supposing the rate of marine denudation 

 to be ten times greater than the above, still we would have an ad- 

 dition of only 15 feet : an amount so insignificant as scarcely worthy 

 of being taken into account in our rough estimate. 



Second : Coral-reefs and limestones formed in the sea. From 

 whence come the materials which go to make these formations? 

 Is it not probable that the greater part of these materials are carried 

 down in solution by rivers from the land ? The sea, no doubt, has 

 its calcareous springs, but so has the land. But the land has more 

 than springs. Eain water, doubtless, washes into rivers far more 

 calcareous materials than is supplied by springs. And as the 

 country is being denuded, new surfaces are being continually ex- 

 posed to the action of the water. But not so in regard to the sea; 

 there springs seem to be the only source of supply. 



Third : What is the amount of materials supplied by submarine and 

 other volcanoes which deposit their materials directly into the ocean ? 

 No one is better qualified to answer this question than Mr. Poulett 

 Serope himself, and it would be desirable if he would turn his atten- 

 tion to this point, and endeavour to arrive at some estimate, however 

 rough, as to the absolute amount. Without some positive knowledge 

 on this point, one is very apt to be misled when he endeavours to 

 compare the amount of materials supplied by this means with that 

 supplied by sub-aerial or by marine denudation. We have a strik- 

 ing example of this in the case of the comparison of the rate of sub- 

 1 See PhU. Mag. for May, 1868, p. 383. 



