siderable portion of this land was elevated, such
a condition of glaciation would certainly be brought about, and would be
heightened whenever a high degree of excentricity prevailed.

It is now the general opinion of geologists that the great continents have
undergone a process of development from earlier to later times. Professor
Dana appears to have been the first who taught it explicitly in the case of
the North American continent, and he has continued the development of his
views from 1856, when he discussed the subject in the _American Journal_,
to the later editions of his _Manual of Geology_ in which the same views
are extended to all the great continents. He says:--

"The North American continent, which since early {95} time had been
gradually expanding in each direction from the northern Azoic, eastward,
westward, and southward, and which, after the Palaeozoic, was finished in
its rocky foundation, excepting on the borders of the Atlantic and Pacific
and the area of the Rocky Mountains, had reached its full expansion at the
close of the Tertiary period. The progress from the first was uniform and
systematic: the land was at all times simple in outline; and its
enlargement took place with almost the regularity of an exogenous
plant."[29]

A similar development undoubtedly took place in the European area, which
was apparently never so compact and so little interpenetrated by the sea as
it is now, while Europe and Asia have only become united into one unbroken
mass since late Tertiary times.

If, however, the greater continents have become more compact and massive
from age to age, and have received their chief extensions northward at a
comparatively recent period, while the Antarctic lands had a corresponding
but somewhat earlier development, we have all the conditions requisite to
explain the persistence, with slight fluctuations, of warm climates far
into the north-polar area throughout Palaeozoic, Mesozoic, and Tertiary
times. At length, during the latter part of the Tertiary epoch, a
considerable elevation took place, closing up several of the water passages
to the north, and raising up extensive areas in the Arctic regions to
become the receptacle of snow and ice-fields. This elevation is indicated
by the abundance of Miocene and the absence of Pliocene deposits in the
Arctic zone and the considerable altitude of many Miocene rocks in Europe
and North America; and the occurrence at this time of a long-cont