478 Correspondence—Mr. Alfred R. Wallace. 
among metamorphic rocks. that various structures, which can only be 
due to the deposition of the original materials, are by no means un- 
frequent, especially among the higher groups. But while thus 
defending myself from an implied charge of error, I am glad to take 
the opportunity of expressing my full concurrence in the general 
conclusions of these valuable papers. They contain a very lucid 
exposition of the principles on which mountain chains have been 
produced, and I expect that in the main the ‘Secret of the High- 
lands’ has been discovered. The analogies between the Highlands 
and the Alps are in many respects close ; but their ‘ mountain making’ 
belongs to very different epochs of geological time, so that they are 
in very different stages of their history. In each we have a great 
nucleus of Archean rocks containing more than one group. In the 
Alps the next great period of deposit on record (I do not forget 
the Carboniferous strata of the west) was throughout the Mesozoic, 
continuing to the earlier Kainozoic. In the Highlands the cor- 
responding period was the earlier Paleozoic. As the Alps became 
_a mountain-chain in Pre-Miocene days, so did the Highlands in Pre- 
Devonian. There is also a close analogy between the Old Red 
Sandstone of the latter and the Nagelflue and Molasse of the 
former. Possibly we may even extend our comparison to the marginal 
volcanic deposits of the two; but be this as it may, there is J think 
little doubt that to interpret the Highlands as a greatly denuded 
mountain-chain is the most hopeful way out of the puzzles which 
their rocks afford. T. G. Bonney. 
MR. WALLACE’S REPLY TO MR. T. MELLARD READE ON THE 
AGEK OF THE EARTH. 
Srr,—I have just received from Mr. T. Mellard Reade, F.G.S., a 
copy of his paper on the ‘‘ Age of the Earth” (which appeared in 
your Magazine of July last), in which I am asked to put that 
gentleman right as regards what he calls his “analysis”’ of some 
figures and estimates given in my “Island Life”; and I gladly 
seize the first opportunity of doing so. To avoid the necessity of 
repeating my own statements as well as those of Mr. Reade, I must 
ask the reader who is interested in this matter to refer back to the 
above-mentioned article. 
The first statement of Mr. Reade’s which I have to “ put right” is 
the following :— ‘It is evident, if the figures mean anything at all, 
that three 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 9526 feet thick deposited 
in all geological time.” This is not quite an accurate representation 
of my statements. The figures quoted represent, not the whole 
matter denuded, but only that portion of which a record still exists 
in the rocks; and this matter has been deposited, not ‘in all 
geological time,” but only in that portion of geological time in- 
dicated by the known series of stratified rocks ; unconformities and 
other breaks representing unknown intervals of which we have no 
