Post-glacial Strata containing Remains of Mastodon giganteus
in North America.
Scarcity of Marine Shells in Glacial Drift of Canada and the
United States.
Greater southern Extension of Ice-action in North America
than in Europe.
Trains of Erratic Blocks of vast Size in Berkshire, Massachusetts.
Description of their Linear Arrangement and Points of Departure.
Their Transportation referred to Floating and Coast Ice.
General Remarks on the Causes of former Changes of Climate at
successive geological Epochs.
Supposed Effects of the Diversion of the Gulf Stream in a
Northerly instead of North-Easterly Direction.
Development of extreme Cold on the opposite Sides of the Atlantic
in the Glacial period not strictly simultaneous.
Effect of Marine Currents on Climate.
Pleistocene Submergence of the Sahara.
On the North American continent, between the arctic circle and the 42nd
parallel of latitude, we meet with signs of ice-action on a scale as
grand as, if not grander than, in Europe; and there also the excess of
cold appears to have been first felt at the close of the Tertiary, and
to have continued throughout a large portion of the Pleistocene period.
[36]
The general absence of organic remains in the North American glacial
formation makes it as difficult as in Europe to determine what mammalia
lived on the continent at the time of the most intense refrigeration, or
when extensive areas were becoming strewed over with glacial drift and
erratic blocks, but it is certain that a large proboscidean now extinct,
the Mastodon giganteus, Cuv., together with many other quadrupeds, some
of them now living and others extinct, played a conspicuous part in the
post-glacial era. By its frequency as a fossil species, this pachyderm
represents the European Elephas primigenius, although the latter also
occurs fossil in the United States and Canada, and abounds, as I learn
from Sir John Richardson, in latitudes farther north than those to which
the mastodon has been traced.
In the state of New York, the mastodon is not unfrequently met with
in bogs and lacustrine deposits formed in hollows in the drift, and
therefore, in a geological position, much resembling that of Recent
peat and shell-marl in the British Isles, Denmark, or the valley of
the Somme, as before described. Sometimes entire skeletons have been
discovered within a few feet of the surface, in peaty earth at the
bottom of small ponds, which the agriculturists had drained. The shells
in these cases belong to freshwater genera, such as Limnaea, Physa,
Planorbis, Cyclas, and others, differing from European species, but
the same as those now proper to ponds and lakes in the same parts of
America.
I have elsewhere given an account of several of these localities which
I visited in 1842,* and can state that they certainly have a more modern
aspect than almost all the European deposits in which remains of the
mammoth occur, although a few instances are cited of Elephas Primigenius
having been dug out of peat in Great Britain.
(* "Travels in North America" volume 1 page 55 London 1845;
and "Manual of Geology" chapter 12 5th edition page 144.)
Thus I was shown a mammoth's tooth in the museum at Torquay in
Devonshire which is believed to have been dredged up from a deposit
of vegetable matter now partially submerged beneath the sea. A more
elevated part of the same peaty formation constitutes the bottom of
the valley in which Tor Abbey stands. This individual elephant must
certainly have been of more modern date than his fellows found fossil in
the gravel of the Brixham cave, before described, for it flourished when
the physical geography of Devonshire, unlike that of the cave period,
was almost identical with that now established.
I cannot help suspecting that many tusks and teeth of the mammoth, said
to have been found in peat, may be as spurious as are the horns of
the rhinoceros cited more than once in the "Memoirs of the Wernerian
Society" as having been obtained from shell-marl in Forfarshire and
other Scotch counties; yet, between the period when the mammoth was
most abundant and that when it died out, there must have elapsed a long
interval of ages when it was growing more and more scarce; and we may
expect to find occasional stragglers buried in deposits long subsequent
in date to others, until at last we may succeed in tracing a passage
from the Pleistocene to the Recent fauna, by geological monuments, which
will fill up the gap before alluded to as separating the era of the
flint tools of Amiens and Abbeville from that of the peat of the valley
of the Somme.
How far the lacustrine strata of North America above mentioned may help
to lessen this hiatus, and whether some individuals of the Mastodon
giganteus may have come down to the confines of the historical period,
is a question not so easily answered as might at first sight be
supposed. A geologist might naturally imagine that the fluviatile
formation of Goat Island, seen at the falls of Niagara, and at several
points below the falls,* was very modern, seeing that the fossil shells
contained in it are all of species now inhabiting the waters of the
Niagara, and seeing also that the deposit is more modern than the
glacial drift of the same locality.
(* "Travels in North America" by the author, volume 1
chapter 2 and volume 2 chapter 19.)
In fact, the old river bed, in which bones of the mastodon occur, holds
the same position relatively to the boulder formation as the strata
of shell-marl and bog-earth with bones of mastodon, so frequent in
the State of New York, bear to the glacial drift, and all may be of
contemporaneous date. But in the case of the valley of the Niagara
we happen to have a measure of time which is wanting in the other
localities, namely, the test afforded by the recession of the falls, an
operation still in progress, by which the deep ravine of the Niagara,
7 miles long, between Queenstown and Goat Island has been hollowed out.
This ravine is not only post-glacial, but also posterior in date to
the fluviatile or mastodon-bearing beds. The individual therefore found
fossil near Goat Island flourished before the gradual excavation of the
deep and long chasm, and we must reckon its antiquity, not by thousands,
but by tens of thousands of years, if I have correctly estimated
the minimum of time which was required for the erosion of that great
ravine.*
(* "Principles of Geology" 9th edition page 2; and "Travels
in North America" volume 1 page 32 1845.)
The stories widely circulated of bones of the mastodon having been
observed with their surfaces pierced as if by arrow-heads or bearing
the marks of wounds inflicted by some stone implement, must in future
be more carefully inquired into, for we can scarcely doubt that the
mastodon in North America lived down to a period when the mammoth
co-existed with Man in Europe. But I need say no more on this subject,
having already explained my views in regard to the evidence of the
antiquity of Man in North America when treating of the human bone
discovered at Natchez on the Mississippi.
In Canada and the United States we experience the same difficulty as
in Europe when we attempt to distinguish between glacial formations
of submarine and those of supra-marine origin. In the New World, as in
Scotland and England, marine shells of this era have rarely been traced
higher than 500 feet above the sea, and 700 feet seems to be the maximum
to which at present they are known to ascend. In the same countries,
erratic blocks have travelled from north to south, following the same
direction as the glacial furrows and striae imprinted almost everywhere
on the solid rocks underlying the drift. Their direction rarely deviates
more than fifteen degrees east or west of the meridian, so that we can
scarcely doubt, in spite of the general dearth of marine shells, that
icebergs floating in the sea and often running aground on its rocky
bottom were the instruments by which most of the blocks were conveyed to
southern latitudes.
There are, nevertheless, in the United States, as in Europe, several
groups of mountains which have acted as independent centres for the
dispersion of erratics, as, for example, the White Mountains, latitude
44 degrees north, the highest of which, Mount Washington, rises to about
6300 feet above the sea; and according to Professor Hitchcock some of
the loftiest of the hills of Massachusetts once sent down their glaciers
into the surrounding lower country.
GREAT SOUTHERN EXTENSION OF TRAINS OF ERRATIC BLOCKS IN BERKSHIRE,
MASSACHUSETTS, U.S., LATITUDE 42 DEGREES NORTH.
Having treated so fully in this volume of the events of the glacial
period, I am unwilling to conclude without laying before the reader the
evidence displayed in North America of ice-action in latitudes farther
south by about ten degrees than any seen on an equal scale in Europe.
This extension southwards of glacial phenomena in regions where there
are no snow-covered mountains like the Alps to explain the exception,
nor any hills of more than moderate elevation, constitutes a feature of
the western as compared to the eastern side of the Atlantic, and must be
taken into account when we speculate on the causes of the refrigeration
of the northern hemisphere during the Pleistocene period.
[Illustration: Figure 50. Erratic Blocks In Berkshire, Massachusetts]
(FIGURE 50. MAP SHOWING THE RELATIVE POSITION AND DIRECTION
OF SEVEN TRAINS OF ERRATIC BLOCKS IN BERKSHIRE, MASSACHUSETTS,
AND IN PART OF THE STATE OF NEW YORK.
Distance in a straight line, between the mountain ranges
A and C, about eight miles.
A. Canaan range, in the State of New York. The crest consists of
green chloritic rock.
B. Richmond range, the western division of which consists in
Merriman's Mount of the same green rock as A, but in a more
schistose form, while the eastern division is composed of
slaty limestone.
C. The Lenox range, consisting in part of mica-schist, and in some
districts of crystalline limestone.
d. Knob in the range A, from which most of the train Number 6 is
supposed to have been derived.
e. Supposed starting point of the train Number 5 in the range A.
f. Hiatus of 175 yards, or space without blocks.
g. Sherman's House.
h. Perry's Peak.
k. Flat Rock.
l. Merriman's Mount.
m. Dupey's Mount.
n. Largest block of train, Number 6. See Figures 51 and 52.
p. Point of divergence of part of the train Number 6, where a
branch is sent off to Number 5.
Number 1. The most southerly train examined by Messrs. Hall and
Lyell, between Stockbridge and Richmond, composed of blocks
of black slate, blue limestone and some of the green Canaan
rock, with here and there a boulder of white quartz.
Number 2. Train composed chiefly of large limestone masses, some
of them divided into two or more fragments by natural joints.
Number 3. Train composed of blocks of limestone and the green
Canaan rock; passes south of the Richmond Station on the Albany
and Boston railway; is less defined than Numbers 1 and 2.
Number 4. Train chiefly of limestone blocks, some of them thirty
feet in diameter, running to the north-west of the Richmond
Station, and passing south of the Methodist Meeting-house,
where it is intersected by a railway cutting.
Number 5. South train of Dr. Reid, composed entirely of large
blocks of the green chloritic Canaan rock; passes north of
the Old Richmond Meeting-house, and is three-quarters of a mile
north of the preceding train (Number 4).
Number 6. The great or principal train (north train of Dr. Reid),
composed of very large blocks of the Canaan rock, diverges at p,
and unites by a branch with train Number 5.
Number 7. A well-defined train of limestone blocks, with a few
of the Canaan rock, traced from the Richmond to the slope
of the Lenox range.)
In 1852, accompanied by Mr. James Hall, state geologist of New York,
author of many able and well-known works on geology and palaeontology,
I examined the glacial drift and erratics of the county of Berkshire,
Massachusetts, and those of the adjoining parts of the state of New
York, a district about 130 miles inland from the Atlantic coast and
situated due west of Boston in latitude 42 degrees 25 minutes north.
This latitude corresponds in Europe to that of the north of Portugal.
Here numerous detached fragments of rock are seen, having a linear
arrangement or being continuous in long parallel trains, running nearly
in straight lines over hill and dale for distances of 5, 10, and 20
miles, and sometimes greater distances. Seven of the more conspicuous
of these trains, from 1 to 7 inclusive, Figure 50, are laid down in the
accompanying map or ground plan.*
(* This ground plan, and a farther account of the Berkshire
erratics was given in an abstract of a lecture delivered by
me to the Royal Institution of Great Britain, April 27, 1855
and published in their Proceedings.)
It will be remarked that they run in a north-west and south-east
direction, or almost transversely to the ranges of hills A, B, and C,
which run north-north-east and south-south-west. The crests of these
chains are about 800 feet in height above the intervening valleys. The
blocks of the northernmost train, Number 7, are of limestone derived
from the calcareous chain B; those of the two trains next to the south,
Numbers 6 and 5, are composed exclusively in the first part of their
course of a green chloritic rock of great toughness, but after they have
passed the ridge B, a mixture of calcareous blocks is observed. After
traversing the valley for a distance of 6 miles these two trains pass
through depressions or gaps in the range C, as they had previously done
in crossing the range B, showing that the dispersion of the erratics
bears some relation to the acutal inequalities of the surface, although
the course of the same blocks is perfectly independent of the more
leading features of the geography of the country, or those by which
the present lines of drainage are determined. The greater number of the
green chloritic fragments in trains 5 and 6 have evidently come from the
ridge A, and a large proportion of the whole from its highest summit d,
where the crest of the ridge has been worn into those dome-shaped
masses called "roches moutonnees," already alluded to, and where several
fragments having this shape, some of them 30 feet long, are seen in
situ, others only slightly removed from their original position, as if
they had been just ready to set out on their travels. Although smooth
and rounded on their tops they are angular on their lower parts, where
their outline has been derived from the natural joints of the rock. Had
these blocks been conveyed from d by glaciers, they would have radiated
in all directions from a centre, whereas not one even of the smaller
ones is found to the westward of A, though a very slight force would
have made them roll down to the base of that ridge, which is very steep
on its western declivity. It is clear, therefore, that the propelling
power, whatever it may have been, acted exclusively in a south-easterly
direction. Professor Hall and I observed one of the green blocks--24
feet long, poised upon another about 19 feet in length. The largest of
all on the west flank of m, or Dupey's Mount, called the Alderman, is
above 90 feet in diameter, and nearly 300 feet in circumference. We
counted at some points between forty and fifty blocks visible at once,
the smallest of them larger than a camel.
[Illustration: Figure 51. Dome-Shaped Block]
(FIGURE 51. ERRATIC DOME-SHAPED BLOCK OF COMPACT CHLORITIC ROCK
(n in map in Figure 50), near the Richmond Meeting-house,
Berkshire, Massachusetts, latitude 42 degrees 25 minutes
North. Length, 52 feet; width, 40 feet; height above the
soil, 15 feet.)
The annexed drawing (Figure 51) represents one of the best known of
train Number 6, being that marked n on the map (Figure 50). According to
our measurement it is 52 feet long by 40 in width, its height above the
drift in which it is partially buried being 15 feet. At the distance
of several yards occurs a smaller block, 3 or 4 feet in height, 20 feet
long, and 14 broad, composed of the same compact chloritic rock, and
evidently a detached fragment from the bigger mass, to the lower and
angular part of which it would fit on exactly. This erratic n has a
regularly rounded top, worn and smoothed like the "roches moutonnees"
before mentioned, but no part of the attrition can have occurred since
it left its parent rock, the angles of the lower portion being quite
sharp and unblunted.
[Illustration: Figure 52. Position of Block in Figure 51]
(FIGURE 52. SECTION SHOWING THE POSITION OF THE BLOCK IN FIGURE 51.
a. The large block in Figure 51 and n in the map in Figure 50.
b. Fragment detached from the same.
c. Unstratified drift with boulders.
d. Silurian limestone in inclined stratification.)
From railway cuttings through the drift of the neighbourhood and other
artificial excavations, we may infer that the position of the block n,
if seen in a vertical section, would be as represented in Figure 52. The
deposit c in that section consists of sand, mud, gravel, and stones,
for the most part unstratified, resembling the till or boulder clay
of Europe. It varies in thickness from 10 to 50 feet, being of greater
depth in the valleys. The uppermost portion is occasionally, though
rarely, stratified. Some few of the imbedded stones have flattened,
polished, striated, and furrowed sides. They consist invariably, like
the seven trains above mentioned, of kinds of rock confined to the
region lying to the north-west, none of them having come from any other
quarter. Whenever the surface of the underlying rock has been exposed by
the removal of the superficial detritus, a polished and furrowed surface
is seen, like that underneath a glacier, the direction of the furrows
being from north-west to south-east, or corresponding to the course of
the large erratics.
As all the blocks, instead of being dispersed from a centre, have
been carried in one direction and across the ridges A, B, C and the
intervening valleys, the hypothesis of glaciers is out of the question.
I conceive, therefore, that the erratics were conveyed to the places
they now occupy by coast ice, when the country was submerged beneath the
waters of a sea cooled by icebergs coming annually from arctic regions.
[Illustration: Figure 53. Canaan And Richmond Valleys]
(FIGURE 53. SECTION THROUGH CANAAN AND RICHMOND VALLEYS AT A TIME
WHEN THEY WERE MARINE CHANNELS.
d, e. Masses of floating ice carrying fragments of rock.)
Suppose the highest peaks of the ridges A, B, C in the annexed diagram
(Figure 53) to be alone above water, forming islands, and d e to be
masses of floating ice, which drifted across the Canaan and Richmond
valleys at a time when they were marine channels, separating islands or
rather chains of islands, having a north-north-east and south-south-west
direction. A fragment of ice such as d, freighted with a block from A,
might run aground and add to the heap of erratics at the north-west base
of the island (now ridge) B, or, passing through a sound between B and
the next island of the same group, might float on till it reached the
channel between B and C. Year after year two such exposed cliffs in the
Canaan range as d and e of the map, Figure 50, undermined by the waves,
might serve as the points of departure of blocks, composing the trains
Numbers 5 and 6. It may be objected that oceanic currents could not
always have had the same direction; this may be true, but during a short
season of the year when the ice was breaking up the prevailing current
may have always run south-east.
If it be asked why the blocks of each train are not more scattered,
especially when far from their source, it may be observed that after
passing through sounds separating islands, they issued again from a
new and narrow starting-point; moreover, we must not exaggerate the
regularity of the trains, as their width is sometimes twice as great
in one place in as another; and Number 6 sends off a branch at p, which
joins Number 5. There are also stragglers, or large blocks here and
there in the spaces between the two trains. As to the distance to which
any given block would be carried, that must have depended on a variety
of circumstances; such as the strength of the current, the direction of
the wind, the weight of the block or the quantity and draught of the ice
attached to it. The smaller fragments would, on the whole, have the best
chance of going farthest; because, in the first place, they were more
numerous, and then, being lighter, they required less ice to float them,
and would not ground so readily on shoals, or if stranded, would be
more easily started again on their travels. Many of the blocks, which at
first sight seem to consist of single masses, are found when examined to
be made up of two, three, or more pieces divided by natural joints. In
the case of a second removal by ice, one or more portions would become
detached and be drifted to different points further on. Whenever this
happened, the original size would be lessened, and the angularity of
the block previously worn by the breakers would be restored, and this
tendency to split may explain why some of the far-transported fragments
remain very angular.
These various considerations may also account for the fact that the
average size of the blocks of all the seven trains laid down on the
plan, Figure 50, lessens sensibly in proportion as we recede from the
principal points of departure of particular kinds of erratics, yet not
with any regularity, a huge block now and then recurring when the rest
of the train consists of smaller ones.
All geologists acquainted with the district now under consideration are
agreed that the mountain ranges A, B, and c, as well as the adjoining
valleys, had assumed their actual form and position before the drift and
erratics accumulated on and in them and before the surface of the fixed
rocks was polished and furrowed. I have the less hesitation in ascribing
the transporting power to coast-ice, because I saw in 1852 an angular
block of sandstone, 8 feet in diameter, which had been brought down
several miles by ice only three years before to the mouth of the
Petitcodiac estuary, in Nova Scotia, where it joins the Bay of Fundy;
and I ascertained that on the shores of the same bay, at the South
Joggins, in the year 1850, much larger blocks had been removed by
coast-ice, and after they had floated half a mile, had been dropped in
salt water by the side of a pier built for loading vessels with coal,
so that it was necessary at low tide to blast these huge ice-borne
rocks with gunpowder in order that the vessels might be able to draw up
alongside the pier. These recent exemplifications of the vast carrying
powers of ice occurred in latitude 46 degrees north (corresponding to
that of Bordeaux), in a bay never invaded by icebergs.
I may here remark that a sheet of ice of moderate thickness, if it
extend over a wide area, may suffice to buoy up the largest erratics
which fall upon it. The size of these will depend, not on the intensity
of the cold but on the manner in which the rock is jointed, and the
consequent dimensions of the blocks into which it splits when falling
from an undermined cliff.
When I first endeavoured in the "Principles of Geology" in 1830,* to
explain the causes, both of the warmer and colder climates which have
at former periods prevailed on the globe, I referred to successive
variations in the height and position of the land and its extent
relatively to the sea in polar and equatorial latitudes--also to
fluctuations in the course of oceanic currents and other geographical
conditions, by the united influence of which I still believe the
principal revolutions in the meteorological state of the atmosphere at
different geological periods have been brought about.
(* 1st edition chapter 7; 9th edition ibid.)
The Gulf Stream was particularly alluded to by me as moderating the
winter climate of northern Europe and as depending for its direction
on temporary and accidental peculiarities in the shape of the land,
especially that of the narrow Straits of Bahama, which a slight
modification in the earth's crust would entirely alter.
Mr. Hopkins, in a valuable essay on the causes of former changes of
climate,*nhas attempted to calculate how much the annual temperature of
Europe would be lowered if this Gulf Stream were turned in some other
and new direction, and estimates the amount at about six or seven
degrees of Fahrenheit.
(* Hopkins, "Quarterly Journal of the Geological Society"
volume 8 1852 page 56.)
He also supposes that if at the same time a considerable part of
northern and central Europe were submerged, so that a cold current from
the arctic seas should sweep over it, an additional refrigeration of
three or four degrees would be produced. He has speculated in the
same essay on the effects which would be experienced in the eastern
hemisphere if the same mighty current of warm water, instead of crossing
the Atlantic, were made to run northwards from the Gulf of Mexico
through the region now occupied by the valley of the Mississippi, and so
onwards to the arctic regions.
After reflecting on what has been said in the thirteenth chapter of
the submergence and re-elevation of the British Isles and the adjoining
parts of Europe, and the rising and sinking of the Alps and the
basins of some of the great rivers flowing from that chain, since the
commencement of the glacial period, a geologist will not be disposed to
object to the theory above adverted to, on the score of its demanding
too much conversion of land into sea, or almost any amount of
geographical change in Pleistocene times. But a difficulty of another
kind presents itself. We have seen that, during the glacial period, the
cold in Europe extended much farther south than it does at present, and
in this chapter we have demonstrated that in North America the cold also
extended no less than 10 degrees of latitude still farther southwards
than in Europe; so that if a great body of heated water, instead of
flowing north-eastward, were made to pass through what is now the centre
of the American continent towards the Arctic Circle, it could not
fail to mitigate the severity of the winter's cold in precisely those
latitudes where the cold was greatest and where it has left monuments
of ice-action surpassing in extent any exhibited on the European side of
the ocean.
In the actual state of the globe, the isothermal lines, or lines of
equal winter temperature when traced westward from Europe to North
America bend 10 degrees south, there being a marked excess of winter
cold in corresponding latitudes west of the Atlantic. During the glacial
period, viewing it as a whole, we behold signs of a precisely similar
deflection of these same isothermal lines when followed from east to
west; so that if, in the hope of accounting for the former severity of
glacial action in Europe, we suppose the absence of the Gulf Stream and
imagine a current of equivalent magnitude to have flowed due north from
the Gulf of Mexico, we introduce, as we have just hinted, a source
of heat into precisely that part of the continent where the extreme
conditions of refrigeration are most manifest. Viewed in this light, the
hypothesis in question would render the glacial phenomena described in
the present chapter more perplexing and anomalous than ever. But here
another question arises, whether the eras at which the maximum of
cold was attained on the opposite sides of the Atlantic were really
contemporaneous? We have now discovered not only that the glacial period
was of vast duration, but that it passed through various phases and
oscillations of temperature; so that, although the chief polishing and
furrowing of the rocks and transportation of erratics in Europe and
North America may have taken place contemporaneously, according to the
ordinary language of geology, or when the same testacea and the
same Pleistocene assemblage of mammalia flourished, yet the extreme
development of cold on the opposite sides of the ocean may not have been
strictly simultaneous, but on the contrary the one may have preceded or
followed the other by a thousand or more than a thousand centuries.
It is probable that the greatest refrigeration of Norway, Sweden,
Scotland, Wales, the Vosges, and the Alps coincided very nearly in time;
but when the Scandinavian and Scotch mountains were encrusted with a
general covering of ice, similar to that now enveloping Greenland, this
last country may not have been in nearly so glacial a condition as now,
just as we find that the old icy crust and great glaciers, which
have left their mark on the mountains of Norway and Sweden, have
now disappeared, precisely at a time when the accumulation of ice in
Greenland is so excessive. In other words, we see that in the present
state of the northern hemisphere, at the distance of about 1500 miles,
two meridional zones enjoying very different conditions of temperature
may co-exist, and we are, therefore, at liberty to imagine some former
alternations of colder and milder climates on the opposite sides of the
ocean throughout the Pleistocene era of a compensating kind, the cold on
the one side balancing the milder temperature on the other. By assuming
such a succession of events we can more easily explain why there has not
been a greater extermination of species, both terrestrial and aquatic,
in polar and temperate regions during the glacial epoch, and why so many
species are common to pre-glacial and post-glacial times.
The numerous plants which are common to the temperate zones north and
south of the equator have been referred by Mr. Darwin and Dr. Hooker to
migrations which took place along mountain chains running from north to
south during some of the colder phases of the glacial epoch.*
(* Darwin, "Origin of Species" chapter 11 page 365; Hooker,
"Flora of Australia" Introduction page 18 1859.)
Such an hypothesis enables us to dispense with the doctrine that the
same species ever originated independently in two distinct and distant
areas; and it becomes more feasible if we admit the doctrine of the
co-existence of meridional belts of warmer and colder climate, instead
of the simultaneous prevalence of extreme cold both in the eastern and
western hemisphere. It also seems necessary, as colder currents of water
always flow to lower latitudes, while warmer ones are running towards
polar regions, that some such compensation should take place, and that
an increase of cold in one region must to a certain extent be balanced
by a mitigation of temperature elsewhere.
Sir John F. Herschel, in his recent work on "Physical Geography," when
speaking of the open sea which is caused in part of the polar regions
by the escape of ice through Behring's Straits, and the flow of warmer
water northwards through the same channel, observes that these straits,
by which the continents of Asia and North America are now parted, "are
only thirty miles broad where narrowest and only twenty-five fathoms
in their greatest depth." But "this narrow channel," he adds, "is yet
important in the economy of nature, inasmuch as it allows a portion
of the circulating water from a warmer region to find its way into the
polar basin, aiding thereby not only to mitigate the extreme rigour of
the polar cold, but to prevent in all probability a continual accretion
of ice, which else might rise to a mountainous height."*
(* Herschel's "Physical Geography" page 41 1861.)
Behring's Straits, here alluded to, happen to agree singularly in width
and depth with the Straits of Dover, the difference in depth not being
more than 3 or 4 feet; so that at the rate of upheaval, which is now
going on in many parts of Scandinavia, of 2 1/2 feet in a century, such
straits might be closed in 3000 years, and a vast accumulation of ice to
the northward commence forthwith.
But, on the other hand, although such an accumulation might spread
its refrigerating influence for many miles southwards beyond the new
barrier, the warm current which now penetrates through the straits, and
which at other times is chilled by floating ice issuing from them, would
when totally excluded from all communication with the icy sea have its
temperature raised and its course altered, so that the climate of some
other area must immediately begin to improve.
There is still another probable cause of a vast change in the
temperature of central Europe in comparatively modern times, to which no
allusion has yet been made; namely, the conversion of the great
desert of the Sahara from sea into land since the commencement of
the Pleistocene period. When that vast region was still submerged, no
sirocco blowing for days in succession carried its hot blasts from a
wide expanse of burning sand across the Mediterranean. The south winds
were comparatively cool, allowing the snows of the Alps to augment to an
extent which the colossal dimensions of the moraines of extinct glaciers
can alone enable us to estimate.
The scope and limits of this volume forbid my pursuing these
speculations and reasonings farther; but I trust I have said enough
to show that the monuments of the glacial period, when more thoroughly
investigated, will do much towards expanding our views as to the
antiquity of the fauna and flora now contemporary with Man, and will
therefore enable us the better to determine the time at which Man began
in the northern hemisphere to form part of the existing fauna. [37]