Sept. 2, 1886] 
will, however, be necessary, as Wallace well insists, that we 
shall hold to that dégree of fixity of the continents in their posi- 
tion, notwithstanding the submergences and emergences they 
have experienced, to which I have already adverted. Sir 
Charles Lyell, more than forty years ago, published in his 
‘*Principles of Geology ” two imaginary maps which illustrate 
the extreme effects of various distribution of land and water. In 
one all the continental masses are grouped around the equator. 
In the other they are all placed around the poles, leaving 
an open equatorial ocean. In the one case the whole of the 
land and its inhabitants would enjoy a perpetual summer, and 
scarcely any ice could exist in the sea. In the other the whole 
of the land would be subjected to an Arctic climate, and 
it would give off immense quantities of ice to cool the ocean. 
But Lyell did not suppose that any such distribution as that 
represented in his maps had actually occurred, though this sup- 
position has been sometimes attributed to him. He merely put 
what he regarded as an extreme case to illustrate what might 
occur under conditions less exaggerated. Sir Charles, like other 
thoughtful geologists, was well aware of the general fixity of the 
areas of the continents, though with great modifications in the 
matter of submergence and of land conditions. The union, 
indeed, of these two great principles of fixity and diversity of the 
continents lies at the foundation of theoretical geology. 
We can now more precisely indicate this than was possible 
when Lyell prodtced his ‘‘ Principles,” and can reproduce the 
conditions of our continents in even the more ancient periods of 
their history. Some examples may be taken from the history of 
the American continent, which is more simple in its arrange- 
ments than the double continent of Europ-asia. We may 
select the early Devonian or Erian period, in which the magni- 
ficent flora of that age—the earliest certainly known to us— 
made its appearance. Imagine the whole interior plain of 
North America submerged, so that the continent is reduced to 
two strips on the east and west, connected by a belt of Lau- 
rentian land on the north. Inthe great mediterranean sea thus 
produced, the tepid water of the equatorial current circulated, 
and it swarmed with corals, of which we know no less than 150 
species, and with other forms of life appropriate to warm seas. 
On the islands and coasts of this sea was introduced the Erian flora, 
appearing first in the north; and with that vitality and colonis- 
ing power of which, as Hooker has well shown, the Scandi- 
navian flora is the best modern type, spreading itself to the 
south. A very similar distribution of land and water in the 
Cretaceous age gave a warm and equable climate in those por- 
tions of North America not submerged, and coincided with 
the appearance of the multitude of broad-leaved trees of 
modern types introduced in the early and middle Cretaceous, 
and which prepared the way for the mammalian life of the 
Eocene. We may take a still later instance from the second 
continental period of the later Pleistocene or early modern, 
when there would seem to have been a partial or entire closure 
of the North Atlantic against the Arctic ice, and wide exten- 
sions seaward of the European and American land, with possibly 
considerable tracts of Jand in the vicinity of the equator, while 
the Mediterranean and the Gulf of Mexico were deep inland 
lakes (Dawkins, Popular Science Monthly, 1873). The effect of 
such conditions on the climates of the northern hemisphere must 
have been prodigious, and their investigation is rendered all the 
more interesting because it would seem that this continental 
period of the post-Glacial age was that in which man made his 
first acquaintance with the coasts of the Atlantic, and possibly 
made his way across its waters. 
We have in America ancient periods of cold as well as of 
warmth. I have elsewhere referred to the boulder conglomer- 
ates of the Huronian, of the Cambrian and Ordovician, of the 
Millstone-grit period of the Carboniferous and of the early 
Permian, but would not venture to affirm that either of these 
periods was comparable in its cold with the later Glacial age, 
still less with that imaginary age of continental glaciation 
assumed by certain of the more extreme theorists (‘* Notes on 
Post-Pliocene of Canada”; ‘‘Pre-Cambrian Glaciers,” Geo/. 
Mag., 1880). These ancient conglomerates were probably pro- 
duced by floating ice, and this at periods when in areas not very 
remote temperate floras and faunas could flourish. The glacial 
periods of our old continent occurred in times when the surface 
* As I have elsewhere endeavoured to show (‘‘ Report on Silurian and 
Devonian Plants of Canada’’), a warm climate in the Arctic region seems 
to have afforded the necessary conditions for the great colonising floras of 
all geological periods. Gray kad previously illustrated the same fact in the 
case of the more modern floras. 
NALTUORE 
417 
of the submerged land was opened up to the northern currents, 
drifting over it mud and sand and stones, and rendering 
nugatory, in so far at least as the bottom of the s2a was con- 
cerned, the effects of the superficial warm streams. Some of 
these beds are also peculiar to the eastern margin of the conti- 
nent, and indicate ice-drift along the Atlantic coast in the same 
manner as at present, while conditions of greater warmth existed 
in the interior. Even in the more recent Glacial age, while the 
mountains were covered with snow and the lowlands submerged 
under a sea laden with ice, there were interior tracts in some- 
what high latitudes of America in which hardy forest trees and 
herbaceous plants flourished abundantly ; and these were by no 
means exceptional “interglacial” peiods. Thus we can show 
that, while from the remote Huronian period to the Tertiary the 
American land occupied the same position as at present, and 
while its changes were merely changes of relative level as com- 
pared with the sea, these have so influenced the ocean currents 
as to cause great vicissitudes of climate. 
Without entering on any detailed discussion of that last and 
greatest Glacial period which is best known to us, and is more 
immediately connected with the early history of man and the 
modern animals, it may be proper to make a few general state- 
ments bearing on the relative importance of sea-borne and land 
ice in producing those remarkable phenomena attributable to 
ice action in this period. In considering this question it must 
be borne in mind that the greater masses of floating ice are pro- 
duced at the seaward extremities of land glaciers, and that the 
heavy field-ice of the Arctic regions is not so much a result of 
the direct freezing of the surface of the sea as of the accumula- 
tion of snow precipitated on the frozen surface. In reasoning 
on the extent of ice action, and especially of glaciers in the 
Pleistocene age, it is necessary to keep this fully in view. Now 
inthe formation of glaciers at present—and it would seem also 
in any conceivable former state of the earth—it is necessary that 
extensive evaporation should conspire with great condensation 
of water in the solid form. Such conditions exist In moun- 
tainous regions sufficiently near to the sea, as in Greenland, 
Norway, the Alps, and the Himalayas ; but they do not exist in 
low Arctic lands like Siberia or Grinnell Land, nor in inland 
mountains. It follows that land-glaciation has narrow limits, 
and that we cannot assume the possibility of great confluent or 
continental glaciers covering the interior of wide tracts of land. 
No imaginable increase of cold could render this possible, inas- 
much as there could not be a sufficient influx of vapour to pro- 
duce the necessary condensation ; and the greater the cold, the 
less would be the evaporation. On the other hand, any in- 
creas? of heat would be felt more rapidly in the thawing and 
evaporation of land ice and snow than on the surface of the 
sea. 
Applying these very simple geographical truths to the North 
Atlantic continents, it is easy to perceive that no amount of 
refrigeration could produce a continental glacier, because there 
could not be sufficient evaporation and precipitation to afford 
the necessary snow in the interior. The case of Greenland is 
often referred to, but this is the case of a high mass of cold land 
with sea, mostly open, on both sides of it, giving, therefore, the 
conditions most favourable to precipitation of snow. If Green- 
land were less elevated, or if there were dry plains around it, 
the case would be quite different, as Nares has well shown by 
his observations on the summer verdure of Grinnell Land, which, 
in the immediate vicinity of North Greenland, presents very 
different conditions as to glaciation and climate.’ If the plains 
were submerged, and the Arctic currents allowed free access to 
the interior of the continent of America, it is conceivable that 
the mountainous regions remaining out of water would be 
covered with snow and ice, and there is the best evidence that 
this actually occurred in the Glacial period ; but with the plains 
out of water this would be impossible. We see evidence of this 
at the present day in the fact that in unusually cold winters the 
great precipitation of snow takes place south of Canada, leaving 
the north comparatively bare, while as the temperature becomes 
milder the area of snow deposit moves farther to the north. 
Thus a greater extension of the Atlantic, and especially of its 
cold ice-laden Arctic currents, becomes the most potent cause of 
a glacial age. 
I have long maintained these conclusions on general geo- 
graphical grounds, as well as on the evidence afforded by the 
Pleistocene deposits of Canada ; and in an address the theme of 
I These views have been admirably illustrated by Von Woeikoff in the 
paper already referred to, and in previous geographical papers. 
