s of denudation to be uniformly spread
over the _whole sea-bottom_ instead of over a narrow belt near the coasts,
a supposition entirely opposed to all the known facts, and which had been
shown by Dr. Croll, five years previously, to be altogether erroneous. (See
_Nature_, Vol. XVIII., p. 268, where Professor Haughton's paper is given as
read before the Royal Society.)

[96] See _Geological Magazine_ for 1877, p. 1.

[97] In his reply to Sir W. Thomson, Professor Huxley _assumed_ one foot in
a thousand years as a not improbable rate of deposition. The above estimate
indicates a far higher rate; and this follows from the well-ascertained
fact, that the area of deposition is many times smaller than the area of
denudation.

[98] Dr. Croll and Sir Archibald Geikie have shown that marine denudation
is very small in amount as compared with sub-aerial, since it acts only
locally on the _edge_ of the land, whereas the latter acts over every foot
of the _surface_. Mr. W. T. Blanford argues that the difference is still
greater in tropical than in temperate latitudes, and arrives at the
conclusion that--"If over British India the effects of marine to those of
fresh-water denudation in removing the rocks of the country be estimated at
1 to 100, I believe that the result of marine action will be greatly
overstated" (_Geology and Zoology of Abyssinia_, p. 158, note). Now, as our
estimate of the rate of sub-aerial denudation cannot pretend to any precise
accuracy, we are justified in neglecting marine denudation altogether,
especially as we have no method of estimating it for the whole earth with
any approach to correctness.

[99] Agassiz appears to have been the first to suggest that the principal
epochs of life extermination were epochs of cold; and Dana thinks that two
at least such epochs may be recognised, at the close of the Palaeozoic and
of the Cretaceous periods--to which we may add the last glacial epoch.

[100] This view was, I believe, first put forth by myself in a paper read
before the Geological Section of the British Association in 1869, and
subsequently in an article in _Nature_, Vol. I. p. 454. It was also stated
by Mr. S. B. J. Skertchley in his _Physical System of the Universe_, p. 363
(1878); but we both founded it on what I now consider the erroneous
doctrine that actual glacial epochs recurred each 10,500 years during
periods of high excentricity.

[101] Explication d'une seconde edition de la _Carte Geolog