310 Notices of Memoirs— 
rate of 1 foot in 3,000 years. He finally concludes that the time 
required to produce this thickness of rock, at the present rate of 
denudation and deposition, is only 28 million years. 
That there is a considerable amount of confusion of ideas per- 
vading this estimate, I think is pretty plain; but let us try and see 
by reversing his calculation in what conclusions we are landed. 
It is evident, if the figures mean anything at all, that 3 millions of 
square miles 177,200 feet thick represent the whole of the rock 
removed by denudation in all forms since the geological history of 
the Earth began. Spread this over 57 million square miles of land, 
and we get a deposit 9,326 feet thick, deposited in all geological 
time. But we must not lose sight of the fact that these hypothetic 
sediments represent rocks made and destroyed over and over again, 
how often it would be difficult to determine; but taking the pro- 
portion of igneous rocks exposed on the surface at 3%; of the total 
area of land,! we may safely put it down as at least 12 times; that 
is, each particle of rock on the average has been denuded and laid 
down at least 12 times. From this it follows that the actual thick- 
ness of the sedimentary crust of the earth, if there were no sedimen- 
tary rocks except on site of the present land areas, would be 
=32< = 777 feet. But even Mr. Wallace has to provide for fluctua- 
tions of his continents in some degree, and I believe he will admit 
that a considerable additional area has been alternately land and sea. 
If he will reason it out, mark its boundaries and estimate its area, I 
think he will find it cannot even on his own hypothesis be less than 
double the present land surface of the globe. The average thickness 
over this increased area would therefore be but 388 feet. If Mr. 
Wallace can prove that the average thickness of the sedimentary 
crust of the globe is no more than this, we shall be a long way on 
the road towards accepting both his hypothesis and his ideas of the 
Earth’s age. 
It so happens that I have but just read “Island Life,” and I find 
my views on the subject of Oceans and Continents characterized as 
“hasty and superficial.” Perhaps they are,-and if Mr. Wallace will 
but just put me right in my analysis of his own calculation, 1 may 
be rapidly converted to profounder geological ideas. 
MOPriGHS OE. Morir on wes 
I.—Tue Decay or Rocks Gronocicatty ConsipERED. By T. 
Sterry Hunt, LL.D., F.R.S.° 
HE author, in this paper, presented in a connected form the 
i principal facts in the history of the decay both of crystalline 
silicated rocks, and of limestones or carbonated rocks, by atmo- 
1 See Chemical Denudation in Relation to Geological Times, p. 57. 
2 Dana, who, it seems (see Continents always Continents, ‘‘ Nature,” March, 1881, ~ 
p- 410), preceded Mr. Wallace in many of the views expressed in ‘‘ Island Life,”’ 
estimates the average thickness of the sedimentary rocks in continental areas at 
5 miles = 26,400 feet. 
° Abstract of a paper read by T. Sterry Hunt, LL.D., F.R.S., before the National 
Academy of Science, at its meeting in Washington, April 17, 1883. 
