Correspondence — Mr. G. Poulett Scrope. 189 



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MEAN THICKNESS OF THE SEDIMENTAET ROCKS. 



SiE, — Permit me space for a few remarks on Mr. J. Croll's 

 interesting paper in your last number, on the mean thickness of the 

 sedimentary rocks. 



The leading idea of Mr. Croll's calculations on this subject is, that 

 the sedimentary rocks are loliolly derived from the detritus carried 

 into the ocean by rivers ; and, consequently, that their volume formed 

 in any specified time, can be accurately gauged by a knowledge 

 of the annual amount of sub-aerial denudation. This appears from 

 the subjoined passage (p. 98, ad. fin.). " If we know the rate at 

 which the land is being denuded, then we know with perfect 

 accuracy the rate at which the sedimentary deposits are being 

 formed in the ocean. This is obvious, because all the materials 

 denuded from the land are deposited in the sea ; and what is 

 deposited in the sea is just what comes off the land, with the 

 exception of the small proportion of calcareous matter which may 

 not have been derived from the land, and which in our rough 

 estimate may be left out of accoimt." 



On this I would remark : 1st. That no allowance is here made 

 for the vast quantities of sedimentary matter carried into the sea 

 by littoral erosion. 



2nd. Can it be allowable to leave out of the account as " of such 

 small proportion as to be unworthy of notice, the calcareous matter 

 which may not have been derived from the land," — a category 

 which would include all coral-reef formations, as well as all sedi- 

 ment from the outflow of calcariferous springs at the bottom of the 

 ocean — a combination in which probably the far greater part of the 

 calcareous sedimentary Eocks had their origin. 



3rd. Neither is any notice taken by Mr. CroU of the enormous 

 amount of argillaceous and siliceous matter spread over the bottom 

 of the ocean after its eruption from submarine, as well as from sub- 

 aerial volcanic mouths in islands or in the vicinity of the sea. This 

 matter either subsides where it falls near the vent, or is distributed 

 by waves and currents over the bottom of the ocean. The vast 

 amount of such fragmentary conglomerate or ash which is annually 

 produced and deposited on the floor of the sea may be guessed at 

 from the fact that all known volcanic orifices are in close proximity 

 to the sea, and the probability that a far larger number are in 

 frequent eruption at its bottom, though outwardly unobserved. It 

 is well known that during some violent volcanic eruptions, though 

 lasting but a few days or weeks, areas of ocean several hundred 

 square miles in extent have been covered with floating pumice and 

 ash, while the finer dust has been spread in quantities over still 

 larger spaces ; all which matter must speedily subside to the bottom. 

 Moreover, before a submarine volcano can rear its head above the 

 sea-level, its loose materials have probably been again and again 

 swept away by the waves and currents, to form sedimentary strata 



