still less numerous, but belonging to other families; and that at the
Eocene period, no one is mentioned by the authors who have shewn the
connection between the fossil and living plants, without having any
preconceived idea on the subject.
"Another fact to be noticed, but which likewise probably depends on
the herbaceous nature of these vegetables, and their leaves not being
shed, is the almost complete absence of Monocotyledons, ferns, and
mosses, which establishes, in regard to these families, a very great
difference between the Pliocene flora and that of modern Europe.
"A difference not less important distinguishes this Flora from that of
the most ancient epochs; namely, the absence, in all these formations,
of the family of Ferns, which, on the contrary, furnishes so prominent
a feature in the Miocene epoch. No trace of them occur in Europe in
the Pliocene formations I have enumerated; while the woods of this
family are very abundant in the formations of the West Indies, which
is considered as an epoch at least as recent as the Pliocene
formation, which appears to indicate that at this period the zones of
vegetation were distributed nearly as at present.
"Indeed, in these modern formations of the Antilles, we find among the
fossil woods, the only portions of their vegetables that have hitherto
been collected, specimens which indicate the existence, not only of
numerous and varied palms, but of many other families of the
equatorial zone, such as Lianes, nearly related to Bauhinia and
Menispermeæ, Pisonia, &c. The vegetation of the Antilles had therefore
at this period the characters of the equatorial zone, as in Europe it
had then the characters of the temperate zone.
"Lastly, and to terminate our observations on this Flora of the latter
geological epoch which preceded the present one, we would remark that,
notwithstanding the general analogies which exist between the
vegetables of these formations and those now living in the temperate
regions, no species appears to be identical, at least with the plants
that still grow in Europe; and if, in some rare cases, complete
identity appears to exist, it is between these vegetables and American
species. Thus the Flora of Europe, even at the most recent geological
epoch, was very different from the European Flora of the present
day."--_L'Institut._
_Glacial Theory of the Erratics and Drift of the New
and Old Worlds_.[45]
By Professor L. AGASSIZ.
Footnote 45: _Vide_ Lake Superior, its physical character,
vegetation, and animals. By Professor Louis Agassiz. 1850.
_Glacialists and Antiglacialists.--Erratic basins of Switzerland.--Similar
phenomena observed in other parts of Europe.--Points necessary to be
settled; first, the relation in time and character between the
Northern and the Alpine erratics.--Traced in North America.--Not yet
settled whether any local centres of distribution in America; but
the general cause must have acted in all parts simultaneously.--This
action ceased at 35° north latitude; this incompatible with the
notion of currents.--In both hemispheres a direct reference to the
Polar Regions.--Difficulty_ _as to so extensive formation of Ice,
removed; difficulties on the theory of Currents, the effects
contrary to experience of Water-Action.--Erratic phenomena of Lake
Superior.--The Iceberg theory.--Description of appearances at Lake
Superior.--Drift; contains mud, and is without fossils.--Example of
juxtaposition of stratified and unstratified Drift, at
Cambridge.--Date of these phenomena not fully determined, but
doubtless simultaneous all over the Globe.--The various periods and
kinds of Drift distinguished.--Accompanied by change of level in the
Continent._
So much has been said and written within the last fifteen years upon
the dispersion of erratic boulders and drift, both in Europe and
America, that I should not venture to introduce this subject again, if
I were not conscious of having essential additions to present to those
interested in the investigation of these subjects.
It will be remarked by all who have followed the discussions
respecting the transportation of loose materials over great distances
from the spot where they occurred primitively, that the most minute
and the most careful investigations have been made by those geologists
who have attempted to establish a new theory of their transportation
by the agency of ice.
The part of those who claim currents as the cause of this
transportation has been more generally negative, inasmuch as,
satisfied with their views, they have generally been contented simply
to deny the new theory and its consequences, rather than investigate
anew the field upon which they had founded their opinions. Without
being taxed with partiality, I may, at the outset, insist upon this
difference in the part taken by the two contending parties. For, since
the publication of Sefstroem's paper upon the drift of Sweden, in
which very valuable information is given respecting the phenomena
observed in that peninsula, and the additional data furnished by De
Verneuil and Murchison upon the same country and the plains of Russia,
the classical ground for erratic phenomena has been left almost
untouched by all except the advocates of the glacial theory. I need
only refer to the investigations of M. de Charpentier, Escher, Von
Derlinth and Studer, and more particularly to those extensive and most
minute researches of Professor Guyot in Switzerland, without speaking
of my own and some contributions from visitors,--as the Martins, James
Forbes, and others, to justify my assertion, that no important fact,
respecting the loose materials spread all over Switzerland, has been
added by the advocates of currents since the days of Sanssure, De Luc,
Escher and Von Buch; whilst Professor Guyot has most conclusively
shewn that the different erratic basins in Switzerland are not only
distinct from each other, as was already known before, but that in
each the loose materials are arranged in well-determined regular
order, shewing precise relations to the centres of distribution, from
which these materials originated; an arrangement which agrees in every
particular with the arrangement of loose fragments upon the surface of
any glacier, but which no cause acting convulsively could have
produced.[46]
Footnote 46: A comparison of the maps, shewing the arrangement of
the moraines upon the glacier of the Aar, in my _Système
Glaciaire_, with the maps which Professor Guyot is about to
publish of the distribution of the erratic boulders in
Switzerland, will shew more fully the identity of the two
phenomena.
The results of these investigations are plainly that the boulders
found at a distance from the Central Alps, originated from their
higher summits and valleys, and were carried down at different
successive periods in a regular manner, forming uninterrupted walls
and ridges, which can be traced from their starting-point to their
extreme peripheric distribution.
I have myself shewn that there are such centres of distribution in
Scotland, and England, and Ireland; and these facts have been since
traced in detail in various parts of the British islands by Dr
Buckland, Sir Charles Lyell, Mr Darwin, Mr M'Laren, and Professor
James Forbes, pointing clearly to the main mountain groups as to so
many distinct centres of dispersion of these loose materials.
Similar phenomena have been shewn in the Pyrenees, in the Black
Forest, and in the Vosges, shewing beyond question, that whatever
might have been the cause of the dispersion of erratic boulders, there
are several separate centres of their distribution to be distinguished
in Europe. But there is another question connected with this local
distribution of boulders which requires particular investigation, the
confusion of which with the former has no doubt greatly contributed to
retard our real progress in understanding the general question of the
distribution of erratics.
It is well known that Northern Europe is strewed with boulders,
extending over European Russia, Poland, Northern Germany, Holland, and
Belgium. The origin of these boulders is far north in Norway, Sweden,
Lapland, and Liefland; but they are now diffused over the extensive
plains west of the Ural Mountains. Their arrangement, however, is such
that they cannot be referred to one single point of origin, but only
in a general way to the northern tracts of land which rise above the
level of the sea in the arctic regions. Whether these boulders were
transported by the same agency as those arising from distinct centres,
on the main Continent of Europe, has been the chief point of
discussion. For my own part, I have indeed no doubt that the extreme
consequences to which we are naturally carried by admitting that ice
was also the agent in transporting the northern erratics to their
present positions, has been the chief objection to the view, that the
Alpine boulders have been distributed by glaciers.
It seemed easier to account for the distribution of the northern
erratics by currents; and this view appearing satisfactory to those
who supported it, they at once went further, and opposed the glacial
theory even in those districts where the glaciers seemed to give a
more natural and more satisfactory explanation of the phenomena. To
embrace the whole question it should be ascertained:
_First_, Whether the northern erratics were transported at the same
time as the local alpine boulders, and if not, which of the phenomena
preceded the other; and again, if the same cause acted in both cases,
or if one of the causes can be applied to one series of these
phenomena, and the other cause to the other series. An investigation
of the erratic phenomena in North America seems to me likely to settle
this question, as the northern erratics occur here in an undisturbed
continuation over tracts of land far more extensive than those in
which they have been observed in Europe. For my own part, I have
already traced them from the eastern shores of Nova Scotia, through
New England and the north-western States of North America and the
Canadas as far as the western extremity of Lake Superior, a region
embracing about thirty degrees of longitude. Here, as in Northern
Europe, the boulders evidently originated farther north than their
present location, and have been moved universally in a main direction
from south to north.
From data which are, however, rather incomplete, it can be further
admitted that similar phenomena occur further west across the whole
continent, everywhere presenting the same relations. That is to say,
everywhere pointing to the north as to the region of the boulders,
which generally disappear about latitude 38°.
Without entering at present into a full discussion of any theoretical
views of the subject, it is plain that any theory, to be satisfactory,
should embrace both the extensive northern phenomena in Europe and
North America, and settle the relation of these phenomena to the
well-authenticated local phenomena of Central Europe.
Whether America itself has its special local circumscribed centres of
distribution or not, remains to be seen. It seems, however, from a few
facts observed in the White Mountains, that this chain, as well
as the mountains of north-eastern New York, have not been
exclusively,--and for the whole duration of the transportation of
these materials,--under the influence of the cause which has
distributed the erratics through such wide space over the continent of
North America. But, whether this be the case or not (and I trust local
investigations will soon settle the question), I maintain that the
cause which has transported these boulders in the American continent,
must have acted simultaneously over the whole ground which these
boulders cover, as they present throughout the continent an
uninterrupted sheet of loose materials, of the same general nature,
connected in the same general manner, and evidently dispersed at the
same time.
Moreover, there is no ground, at present, to doubt the simultaneous
dispersion of the erratics over Northern Europe and Northern America.
So that the cause which transported them, whatever it may be, must
have acted simultaneously over the whole tract of land west of the
Ural Mountains, and east of the Rocky Mountains, without assuming
anything respecting Northern Asia, which has not yet been studied in
this respect; that is to say, at the same time, over a space embracing
two hundred degrees of longitude.
Again, the action of this cause must have been such, and I insist
strongly upon this point, as a fundamental one, the momentum with
which it acted must have been such, that after being set in motion in
the north, with a power sufficient to carry the large boulders which
are found everywhere over this vast extent of land, it vanished, or
was stopped, after reaching _the thirty-fifth degree of northern
latitude_.
Now it is my deliberate opinion that natural philosophy and
mathematics may settle the question, whether a body of water of
sufficient extent to produce such phenomena can be set in motion with
sufficient velocity to move all these boulders; and nevertheless stop
before having swept over the whole surface of the globe. Hydrographers
are familiar with the action of currents, with their speed, and with
the power with which they can act. They know also how they are
distributed over the globe. And, if we institute a comparison, it will
be seen that there is nowhere a current running from the poles towards
the lower latitudes, either in the northern or southern hemisphere,
covering a space equal to one-tenth of the currents which should have
existed to carry the erratics into their present position. The widest
current is west of the Pacific, which runs parallel to the equator,
across the whole extent of that sea from east to west, and the
greatest width of which is scarcely fifty degrees. This current, as a
matter of course, establishes a regular rotation between the waters
flowing from the polar regions towards lower latitudes.
The Gulf Stream, on the contrary, runs from west to east, and dies out
towards Europe and Africa, and is compensated by the currents from
Baffin's Bay and Spitzbergen emptying into the Atlantic, while the
current of the Pacific, moving towards Asia, and carrying floods of
water in that direction, is maintained chiefly by antarctic currents,
and those which follow the western shore of America from Behring's
Straits. Wherever they are limited by continents, we see that the
waters of these currents, even when they extend over hundreds of
degrees of latitude, as the Gulf Stream does in its whole course, are
deflected where they cannot follow a straight course.
Now, without appealing with more detail to the mechanical conditions
involved in this inquiry, I ask every unprejudiced mind acquainted
with the distribution of the northern boulders, whether there was any
geographical limitation to the supposed northern current to cause it
to leave the northern erratics of Europe in such regular order, with a
constant bearing from north to south, and to form, on its southern
termination, a wide, regular zone from Asia to the western shores of
Europe, north of the fiftieth degree of latitude, before it had
reached the great barrier of the Alps? I ask, whether there was such a
barrier in the unlimited plains which stretch from the Arctic seas
uninterrupted over the whole northern continent of America as far down
as the Gulf of Mexico?
I ask, again, why the erratics are circumscribed within the northern
limits of the temperate zone, if their transportation is owing to the
action of water currents? Does not, on the contrary, this most
surprising limit within the arctic and northern temperate zones, and,
in the same manner, within the antarctic and southern temperate zones,
distinctly shew that the cause of transportation is connected with the
temperature or climate of the countries over which the phenomena were
produced? If it were otherwise, why are there no systems of erratics
with an east and west bearing, or in the main direction of the most
extensive currents flowing at present over the surface of our globe?
It is a matter of fact, of undeniable fact, for which the theory has
to account, that, in the two hemispheres, the erratics have direct
reference to the polar regions, and are circumscribed within the
arctics and the colder part of the temperate zones. This fact is as
plain as the other fact, that the local distribution of boulders has
reference to high mountain ranges, to groups of land raised above the
level of the sea into heights, the temperature of which is lower than
the surrounding plains. And what is still more astonishing, the extent
of the local boulders, from their centre of distribution, reaches
levels, the mean annual temperature of which corresponds, in a
surprising manner, with the mean annual temperature of the southern
limit of the northern erratics.
We have, therefore, in this agreement, a strong evidence in favour of
the view that both the phenomena of local mountain erratics in Europe,
and of northern erratics in Europe and America, have probably been
produced by the same cause.
The chief difficulty is in conceiving the possibility of the formation
of a sheet of ice sufficiently large to carry the northern erratics
into their present limits of distribution; but this difficulty is
greatly removed when we can trace, as in the Alps, the progress of the
boulders under the same aspect from the glaciers now existing, down
into regions where they no longer exist, but where the boulders and
other phenomena attending their transportation shew distinctly that
they once existed.
Without extending further this argumentation, I would call the
attention of the unprejudiced observer to the fact, that those who
advocate currents as the cause of the transportation of erratics,
have, up to this day, failed to shew, in a single instance, that
currents can produce all the different phenomena connected with the
transportation of the boulders which are observed everywhere in the
Alps, and which are still daily produced there by the small glaciers
yet in existence. Never do we find that water leaves the boulders
which it carries along in regular walls of mixed materials; nor do
currents anywhere produce upon the hard rocks _in situ_ the peculiar
grooves and scratches which we see everywhere under the glacier and
within the limits of their ordinary oscillations.
Water may polish the rocks, but it nowhere leaves straight scratches
upon their surface; it may furrow them, but these furrows are sinuous,
acting more powerfully upon the soft parts of the rocks or fissures
already existing; whilst glaciers smooth and level uniformly the
hardest parts equally with the softest, and, like a hard file, rub to
uniform continuous surfaces the rocks upon which they move.
But now let us return to our special subject, the erratics of North
America.
The phenomena of drift are more complicated about Lake Superior than I
have seen them anywhere else; for, besides the general phenomena which
occur everywhere, there are some peculiarities noticed which are to be
ascribed to the lake as such, and which we do not find in places where
no large sheet of water has been brought into contact with the erratic
phenomena. In the first place, we notice about Lake Superior an
extensive tract of polished, grooved and scratched rocks, which
present here the same uniform character which they have everywhere. As
there is so little disposition, among so many otherwise intelligent
geologists, to perceive the facts as they are, whenever they bear upon
the question of drift, I cannot but repeat, what I have already
mentioned more than once, but what I have observed again here over a
tract of some fifteen hundred miles, that the rocks are everywhere
smoothed, rounded, grooved and furrowed in a uniform direction. The
heterogeneous materials of which the rocks consist are cut to one
continuous uniform level, shewing plainly that no difference in the
polish and abrasion can be attributed to the greater or less
resistance on the part of the rocks, but that a continuous rush cut
down everything, adapting itself, however, to the general undulations
of the country, but nevertheless shewing, in this close adaptation, a
most remarkable continuity in its action.
That the power which produced these phenomena moved in the main from
north to south, is distinctly shewn by the form of the hills, which
present abrupt slopes, rough and sharp corners towards the south,
while they are all smoothed off towards the north.
Indeed, here, as in Norway and Sweden, there is on all the hills a
lee-side and a strike-side. As has been observed in Norway and Sweden,
the polishing is very perfect in many places, sometimes strictly as
brilliant as a polished metallic surface, and everywhere these
surfaces are more or less scratched and furrowed, and both scratches
and furrows are rectilinear, crossing each other under various angles;
however, never varying many points of the compass on the same spot,
but in general shewing that where there are deviations from the most
prominent direction, they are influenced by the undulations of the
soil. It has been said, that the main direction of these striæ was
from north-west to south-east, but I have found it as often strictly
from north to south, or even from north-east to south-west; and if we
are to express a general result, we should say that the direction,
assigned by all our observations to the various scratches, tends to
shew that they have been formed under the influence of a movement from
north to south, varying more or less to the east and west, according
to local influences in the undulations of the soil. It is, indeed, a
very important fact, that scratches which seem to have been produced
at no great intervals from each other, are not absolutely parallel,
but may diverge for ten, fifteen, or more degrees. There is one
feature in these phenomena, however, in which we never observe any
variation. The continuity of these lines is absolutely the same
everywhere. They are rectilinear and continuous, and cannot be better
compared than with the effects of stones or other hard materials
dragged in the same direction upon flat or rolling surfaces; they form
simple scratches extending for yards in straight lines, or breaking
off for a short space to continue again in a straight line in the same
direction, just as if interrupted by a jerk. There are also deeper
scratches of the same kind, presenting the same phenomena, only,
perhaps, traceable for a greater distance than the finer ones. These
scratches, instead of appearing like the tracing of diamonds upon
glass, as the former do, would rather assume the appearance of a
deeper groove, made by the point of a graver, or perhaps still more
closely resemble the scratches which a cart-wheel would produce upon
polished marble, if the wheel were chained, and coarse sand spread
over the floor. The appearance of the rock, crushed by the moving
mass, is especially distinct in limestone rocks, where grooves are
seldom nicely cut, but present the appearance of a violent pressure
combined with the grooving power, thus giving to the groove a
character which is quite peculiar, and which at once strikes an
observer who has been familiar with its characteristic aspect. Now, I
do not know upon what the assertions of some geologists rest, that
gravel, moved by water under strong heavy currents, will produce
similar effects. Wherever I have gone since studying these phenomena,
I have looked for such cases, and have never yet found modern gravel
currents produce any thing more than a smooth surface, with undulating
furrows following the cracks in the rocks, or following their softer
parts; but continuous straight lines, especially such crushed lines
and straight furrows, I have never seen.