proof to the view that the climates of former
periods are not due to any general refrigeration, but to causes which were
subject to change and alternation in former ages as now.

[73] Mr. S. B. J. Skertchley informs me that he has himself observed thick
Tertiary deposits, consisting of clays and anhydrous gypsum, at Berenice on
the borders of Egypt and Nubia, at a height of about 600 feet above the
sea-level; but these may have been of fresh-water origin.

[74] By referring to our map of the Indian Ocean showing the submarine
banks indicating ancient islands (Chap. XIX.), it will be evident that the
south-east trade-winds--then exceptionally powerful--would cause a vast
body of water to enter the deep Arabian Sea.

[75] In his recently published _Lectures on Physical Geography_, Professor
Haughton calculates, that more than half the solar heat of the torrid zone
is carried to the temperate zones by ocean currents. The Gulf Stream itself
carries one-twelfth of the total amount, but it is probable that a very
small fraction of this quantity of heat reaches the polar seas owing to the
wide area over which the current spreads in the North Atlantic. The
corresponding stream of the Indian Ocean in Miocene times would have been
fully equal to the Gulf Stream in heating power, while, owing to its being
so much more concentrated, a large proportion of its heat may have reached
the polar area. But the Arctic Ocean occupies less than one-tenth of the
area of the tropical seas; so that, whatever proportion of the heat of the
tropical zone was conveyed to it, would, by being concentrated into
one-tenth of the surface, produce an enormously increased effect. Taking
this into consideration, we can hardly doubt that the opening of a
sufficient passage from the Indian Ocean to the Arctic seas would produce
the effects above indicated.

[76] For an account of the resemblances and differences of the mammalia of
the two continents during the Tertiary epoch, see my _Geographical
Distribution of Animals_, Vol. I. pp. 140-156.

[77] Professor Haughton has made an elaborate calculation of the difference
between existing climates and those of Miocene times, for all the places
where a Miocene flora has been discovered, by means of the actual range of
corresponding species and genera of plants. Although this method is open to
the objection that the ranges of plants and animals are not determined by
temperature only, yet the results may be appr