O12 Correspondence—Ur. T. Mellard Reade. 
what then have they in addition been derived ? The only other form 
of rocks we know of are the igneous rocks, granites—when not 
metamorphic—syenites, basalts, etc.; but no one can affirm that even 
these are parts of the original non-sedimentary globe; they are no 
doubt largely re-melted sediments. But for my purpose I assume 
that they are, and I find that the area of igneous rocks exposed to 
denuding agencies is about =4; the area of the whole land, and there 
is reason to suppose that this proportion has endured since the 
earliest rocks we know of were formed.! It follows that if all the 
land areas, igneous and sedimentary, were denuded at the same rate 
—as no doubt they have been in the aggregate—the process of accu- 
mulation of the existing thickness of sedimentary rocks has taken 
12 times as long as if they had been derived directly from a bare 
original crust. Now it is only on the latter supposition, which we 
know to be contrary to fact, that Mr. Wallace’s calculation could be 
true in principle or result. 
But whether I was right or wrong in the figures given, it is a fact 
admitted by all geologists since the time of Hutton, that the sedi- 
mentary materials of the globe have been used up over and over 
again, and any calculation of the age of the Earth based upon the 
rate of accumulation of sediments and their aggregate bulk which 
ignores this, as Mr. Wallace’s does, is either incomplete or funda- 
mentally wrong. ' T. Mexuarp .READE. 
Oct. 5th, 1883. 
REPLY TO MR. SKERTCHLY. 
Sir,—Since the above reply to Mr. Wallace was written Mr. 
Skertchly has published a letter in the GeotocicaL Macazinz, on 
the same subject, in which he says, ‘“ First, I fail to see the slightest 
connexion between the area of exposed igneous rocks and the number 
of times sedimentary beds have been ‘ worked over’ again. Surely 
at the beginning of geological time all -the land was igneous, and 
practically that area has been diminishing ever since. This can 
therefore afford no clue to the question.” To which I reply, Geological 
Time is the time of which we have geological knowledge, and Mr. 
Wallace’s calculation as well as my own is limited to that time. The 
earliest recognized system of sedimentary rocks are Laurentian, and 
there is absolutely no data to prove that the igneous areas even in 
this period were greater in proportion to the sedimentary than they are 
now—if there is, I shall be glad to hear it. The hypothetical period 
between the Laurentian and the time when all the land was igneous 
is anterior to the date at which any calculation of the “Age of the 
Earth” based on sedimentation can commence, for there are no data 
on which to work. An inspired seer might perhaps tell us some- 
thing of this period; but as I have no pretence to fill that réle, it is 
useless for me to attempt it. 
Mr. Skertchly also says, “Thirdly, Mr. Reade supposes the denu- 
dation of sedimentary rocks would reduce the mean thickness.” As 
hs This question is discussed in my Chemical Denudation in Relation to Geological 
ime. 
