Correspondence—Mr. Alfred R. Wallace. 479 
record. With these corrections the figures used? by me do imply 
what Mr. Reade says they do; which is, in other words,—that the 
average thickness of that portion of the earth’s crust formed by the 
known stratified rocks does not probably exceed nine or ten thousand 
feet. 
Mr. Reade, however, without directly impugning these figures, 
attempts to show that they lead to absurd or incredible results, and 
he does this by manipulating them in a way which is altogether 
beyond my comprehension. He first says, that these rocks have 
been made and destroyed over and over again ; and then argues that, 
because the exposed igneous rocks cover about. ="; of the land surface, 
therefore ‘‘each particle of rock, on the average, has been denuded 
and laid down at least twelve times.” I have in vain tried to see any 
connection between these two statements, but what follows is still 
more unintelligible. Mr. Reade adds :—“ From this it follows that 
the actual thickness of the sedimentary crust of the earth, if there 
were no sedimentary rocks except on the site of the present land 
areas, would be 532, — 77 feet.” Correcting the clerical error of 
zit instead of 2226, this means that, because the stratified rocks 
have been successively formed from the denudation of older rocks 
(stratified and igneous), therefore their actual thickness would be 
many times less than by estimates founded on direct measurement it 
is known they actually are! It is, I think, evident that, from Mr. 
Reade’s point of view, he should have here multiplied instead of 
divided by 12. For if the older rocks have been reduced in thick- 
ness by denudation, and their débris has gone to form newer rocks 
in each successive epoch, it is clear that when first deposited all the 
rocks would have been thicker than now, though there is no definite 
relation between the number of successive formations and their 
greater thickness, as Mr. Reade seems to suppose. For example, if 
half the original Paleozoic rocks have been denuded to form the 
Mesozoic and parts of the later rocks, and half of the original 
Mesozoic to form the Tertiary, and half these again to form glacial 
and recent deposits, each would have been at first about twice as 
thick as it is now,—not one-fourth the thickness, as Mr. Reade’s mode 
of calculation would make them; and as the whole problem is one of 
the time taken to produce these various deposits, the greater original 
thickness would have to be used in the calculation. 
But, even if Mr. Reade’s figures are thus corrected, his whole 
criticism is radically unsound; for, as I have explained in my 
original discussion of the subject, denudation is so unequal in its 
action and occurs so generally on the edges of uplifted strata not 
over their surfaces of deposit, that it would be quite possible for 3% 
or even 35 of a formation to be destroyed by denudation, and yet 
for the remaining +15 or 73> to give a fairly accurate measure of the 
average thickness or even sometimes of a maximum thickness of the 
original deposit. Our measures of the thickness of the sedimentary 
rocks will, therefore, not be seriously affected by the fact that by far 
the larger portion of all of them have been destroyed by denudation, 
and again and again laid down to form newer rocks; and as I have 
