is on every shore and in every water-current,--when we can trace this
action almost everywhere, and nowhere find it similar to the phenomena
just described, I cannot imagine upon what ground these phenomena are
still attributed to the agency of currents. This is the less rational
as we have at present, in all high mountain chains of the temperate
zone, other agents, the glaciers, producing these very same phenomena,
with precisely the same characters, to which therefore a sound
philosophy should ascribe, at least conditionally, the northern and
alpine polished surfaces, and scratched and grooved rocks, or at least
acknowledge that the effect produced by the action of glaciers more
nearly resembles these erratic phenomena than does that which results
from the action of currents. But such is the prejudice of many
geologists, that those keen faculties of distinction and
generalization, that power of superior perception and discrimination,
which have led them to make such brilliant discoveries in geology in
general, seem to abandon them at once as soon as they look at the
erratics. The objection made by a venerable geologist, that the cold
required to form and preserve such glaciers, for any length of time,
would freeze him to death, is as childish as the apprehension that the
heavy ocean-currents, the action of which he sees everywhere, might
have swept him away.[47]
Footnote 47: Berlin Academy, 1846.
Now that these phenomena have been observed extensively, we may derive
also some instruction from the limits of their geographical extent.
Let us see, therefore, where these polished, scratched, and furrowed
rocks have been observed.
In the first place they occur everywhere in the north within certain
limits of the arctics, and through the colder parts of the temperate
zone. They occur also in the southern hemisphere, within parallel
limits, but in the plains of the tropics, and even in the warmer parts
of the temperate zone we find no trace of these phenomena, and
nevertheless the action of currents could not be less there, and could
not at any time have been less there than in the colder climates. It
is true, similar phenomena occur in Central Europe, and have been
noticed in Central Asia, and even in the Andes of South America, but
these always in higher regions, at definite levels above the surface
of the sea, everywhere indicating a connection between their extent
and the colder temperature of the places over which they are traced.
More recently, a step towards the views I entertain of this subject
has been made by those geologists who would ascribe them to the agency
of icebergs. Here, as in my glacial theory, ice is made the agent;
floating ice is supposed to have ground and polished the surfaces of
rocks, while I consider them to have been acted upon by terrestrial
glaciers. To settle this difference we have a test which is as
irresistible as the other arguments already introduced.
Let us investigate the mode of action, the mode of transportation of
icebergs, and let us examine whether this cause is adequate to produce
phenomena for which it is made to account. As mentioned above, the
polished surfaces are continuous over hills, and in depressions of the
soil, and the scratches which run over such undulating surfaces are
nevertheless continuous in straight lines. If we imagine icebergs
moving upon shoals, no doubt they would scratch and polish the rocks
in a way similar to moving glaciers. But upon such grounds they would
sooner or later be stranded; and if they remained loose enough to
move, they would, in their gyratory movements, produce curved lines,
and mark the spots where they had been stranded with particular
indications of their prolonged action. But nowhere upon arctic ground
do we find such indications. Everywhere the polished and scratched
surfaces are continuous in straight juxtaposition.
Phenomena analogous to those produced by icebergs would only be seen
along the sea-shores; and if the theory of drifted icebergs were
correct, we should have, all over those continents where erratic
phenomena occur, indications of retreating shores as far as the
erratic phenomena are found. But there is no such thing to be observed
over the whole extent of the North American continent, nor over
Northern Europe and Asia, as far as the northern erratics extend. From
the arctics to the southernmost limit of the erratic distribution, we
find nowhere the indications of the action of the sea as directly
connected with the production of the erratic phenomena. And wherever
the marine deposits rest upon the polished surfaces of ground and
scratched rocks, they can be shewn to be deposits formed since the
grooving and polishing of the rocks, in consequence of the subsidence
of those tracts of land upon which such deposits occur.
Again, if we take for a moment into consideration the immense extent
of land covered by erratic phenomena, and view them as produced by
drifted icebergs, we must acknowledge that the icebergs of the
_present period_ at least, are insufficient to account for them, as
they are limited to a narrower zone. And to bring icebergs in any way
within the extent which would answer for the extent of the
distribution of erratics, we must assume that the northern ice-fields,
from which these icebergs could be detached and float southwards, were
much larger at the time they produced such extensive phenomena than
they are now. That is to say, we must assume an ice period; and if we
look into the circumstances, we shall find that this ice period, to
answer to the phenomena, should be nothing less than an extensive cap
of ice upon both poles. This is the very theory which I advocate; and
unless the advocates of an iceberg-theory go to that length in their
premises, I venture to say, without fear of contradiction, that they
will find the source of their icebergs fall short of the requisite
conditions which they must assume, upon due consideration, to account
for the whole phenomena as they have really been observed.
But without discussing any farther the theoretical views of the
question, let me describe more minutely the facts, as observed on the
northern shores of Lake Superior. The polished surfaces, as such, are
even, undulating, and terminate always above the rough lee-side turned
to the south, unless upon gentle declivities, where the polished
surfaces extend in unbroken continuity upon the southern surfaces of
the hills, as well as upon their northern slopes. On their eastern and
western flanks, shallow valleys running east and west are as uniformly
polished as those which run north and south; and this fact is more and
more evident, wherever scratches and furrows are also well preserved
and distinctly seen, and by their bearings we can ascertain most
minutely, the direction of the onward movement which produced the
whole phenomena. Nothing is more striking in this respect than the
valleys or depressions of the soil running east and west, where we see
the scratches crossing such undulations at right angles, descending
along the southern gentle slope of a hill, traversing the flat bottom
below, and rising again up the next hill south, in unbroken
continuity. Examples of the kind can be seen everywhere in those
narrow inlets, with shallow waters intersecting the innumerable
highlands along the northern shores of Lake Superior, where the
scratches and furrows can be traced under water from one shore to the
other, and where they at times ascend steep hills, which they cross at
right angles along their northern slope, even when the southern slope,
not steeper in itself, faces the south with rough escarpments.
The scratches and furrows, though generally running north and south,
and deviating slightly to the east and west, present, in various
places, remarkable anomalies, even in their general course along the
eastern shore of the lake. Between Michipicotin and Sault St Marie, we
more frequently see a deflection to the west than a due north and
south course, which is rather normal along the northern shore proper,
between Michipicotin and other islands, and from the Pic to
Fort-William; the deep depression of the lake being no doubt the cause
of such a deviation, as large masses of ice could accumulate in this
extensive hollow cavity before spreading again more uniformly beyond
its limits. To the oscillations of the whole mass in its southerly
movement, according to the inequalities of the surfaces, we must
ascribe the crossing of the straight lines at acute angles, as we
observe also at the present day under the glaciers, as they swell and
subside, and hence meet with higher and lower obstacles in their
irregular course between the Alpine valleys.
In deep, narrow chasms, however, we find now and then greater
deviations from the normal direction of the striæ, where considerable
masses of ice could accumulate, and move between steep walls under a
lateral pressure of the masses moving onwards from the north. Such a
chasm is seen between Spar Island and the main land opposite Prince's
Location, south of Fort-William, where the furrows and scratches run
nearly east and west. But here also, there is no tumultuous
disturbance in the continuation of the phenomena, such as would occur
if icebergs were floated and stranded against the southern barrier.
The same continuity of even, polished surfaces, with their scratches
and furrows, prevails here as elsewhere. The angles which these
scratches form with each other are very acute, generally not exceeding
10°; but at times they diverge more, forming angles of 15°, 20°, and
25°. In a few instances, I have even found localities where they
crossed each other at angles of no less than 30°; but these are rare
exceptions. It may sometimes be noticed that the lines running in one
direction form a system by themselves, varying very little from strict
parallelism with each other, but crossing another system, more or less
strongly marked, of other lines equally parallel with each other. At
other times, a system of lines, strongly marked and diverging very
slightly, seem to pass over another system, in which the lines form
various angles with each other. Again, there are places,--and this is
the most common case, where the lines diverge slightly, following,
however, generally one main direction, which is crossed by fewer
lines, forming more open angles. These differences, no doubt, indicate
various oscillations in the movement of the mass which produced the
lines, and shew probably its successive action, with more or less
intensity, upon the same point at successive periods, in accordance
with the direction of the moving force at each interval. The same
variations within precisely the same limits may be noticed in our day
on the margin of the glaciers produced by the increase or diminution
of the bulk of their mass, and the changes on the rate of their
movement.
The loose materials which produced, in their onward movement under the
pressure of ice, such polishing and grooving, consisted of
various-sized boulders, pebbles, and gravels, down to the most minute
sand and loamy powder. Accumulations of such materials are found
everywhere upon these smooth surfaces, and in their arrangement they
present everywhere the most striking contrast when compared with
deposits accumulated under the agency of water. Indeed, we nowhere
find this glacial drift regularly stratified, being every where
irregular accumulations of loose materials, scattered at random
without selection, the coarsest and most minute particles being piled
irregularly in larger or smaller heaps, the greatest boulders standing
sometimes uppermost, or in the centre, or in any position among
smaller pebbles and impalpable powder.
And these materials themselves are scratched, polished and furrowed,
and the scratches and furrows are rectilinear as upon the rocks _in
situ_ underneath, not bruised simply, as the loose materials carried
onward by currents or driven against the shores by the tides, but
regularly scratched, as fragments of hard materials would be if they
had been fastened during their friction against each other, just as we
observe them upon the lower surface of glaciers where all the loose
materials are set in ice, as stones in their setting are pressed and
rubbed against underlying rocks. But the setting here being simply
ice, these loose materials, fast at one time and moveable another, and
fixed and loosened again, have rubbed against the rock below in all
possible positions; and hence, not only their rounded form, but also
their rectilinear grooving. How such grooves could be produced under
the action of currents, I leave to the advocates of such a theory to
shew; as soon as they shall be prepared for it.
I should not omit here to mention a fact which, in my opinion, has a
great theoretical importance, namely, that in the northern erratics,
even the largest boulders, as far as I know, are rounded, and
scratched and polished; at least, all those which are found beyond the
immediate vicinity of the higher mountain ranges, shewing that the
accumulations of ice which moved the northern erratics covered the
whole country; and this view is sustained by another set of facts
equally important, namely, that the highest ridges, the highest rugged
mountains, at least, in this continent and north of the Alps in
Europe, are as completely polished and smoothed as the lower lands,
and only a very few peaks seem to have risen above the sheet of ice;
whilst, in the Alps, the summits of the mountains stand generally
above these accumulations of ice, and have supplied the surface of the
glaciers with large numbers of angular boulders, which have been
carried upon the back of glaciers to the lower valleys and adjacent
plains without losing their angular forms.
With respect to the irregular accumulation of drift-materials in the
north, I may add, that there is not only no indication of
stratification among them, such, unquestionably, as water would have
left, but that the very nature of these materials shews plainly that
they are of terrestrial origin; for the mud which sticks between them
adheres to all the little roughnesses of the pebbles, fills them out,
and has the peculiar adhesive character of the mud ground under the
glaciers, and differing entirely in that respect from the gravels, and
pebbles, and sands washed by water-currents, which leave each pebble
clean, and never form adhering masses, unless penetrated by an
infiltration of limestone.
Another important fact respecting this glacial draft consists in the
universal absence of marine, as well as fresh-water fossils in its
interior--a fact which strengthens the view that they have been
accumulated by the agency of strictly terrestrial glaciers; such is,
at least, the case everywhere far from the sea-shore. But we may
conclude that these ancient glaciers reached, upon various points, the
sea-shore at the time of their greatest extension, just as they do at
present in Spitzbergen and other arctic shores; and that therefore, in
such proximity, phenomena of contact should be observed, indicating
the onward movement of glacial material into the ocean, such as the
accumulation within these materials of marine fossil remains, and also
the influence of the tidal movements upon them. And now such is really
the case. Nearer the sea-shores we observe distinctly, in some
accumulations of the drift, faint indications of the action of the
tide, reaching the lower surface of glaciers, and the remodelling to
some extent of the materials which these poured into the sea. A
beautiful example of the kind may be observed near Cambridge, along
Charles River, not far from Mount Auburn, where the unstratified
glacial drift (_a_) presents in its upper masses strictly the
characters of true terrestrial glacial accumulation, but shews
underneath faint indications (_b_) of the action of tides. Above,
regular tidal strata (_c_) are observed, formed probably after the
masses below had subsided. The surface of this accumulation is covered
with soil (_d_).
[Illustration: Sketch of river bank stratification]
The period at which these phenomena took place cannot be fully
determined, nor is it easy to ascertain whether all glacial drift is
contemporaneous. It would seem, however, as if the extensive
accumulation of drift all around the northern pole in Europe, Asia,
and America was of the same age as the erratic of the Alps. The
climatic circumstances capable of accumulating such large masses of
ice around the north pole, having no doubt extended their influence
over the temperate zone, and probably produced, in high mountain
chains, as the Alps, the Pyrenees, the Black Forest, and the Vosges,
such accumulations of snow and ice as may have produced the erratic
phenomena of those districts. But extensive changes must have taken
place in the appearance of the continents over which we trace erratic
phenomena, since we observe in the Old World, as well as in North
America, extensive stratified deposits containing fossils which rest
upon the erratics; and as we have all possible good reasons and
satisfactory evidence for admitting that the erratics were transported
by the agency of terrestrial glaciers, and that, therefore, the tracts
of land over which they occur stood at that time above the level of
the sea, we are led to the conclusion that these continents have
subsided since that period below the level of the sea, and that over
their inundated portions, animal life has spread, remains of organized
beings have been accumulated, which are now found in a fossil state in
the deposits formed under those sheets of water.
Such deposits occur at various levels in different parts of North
America. They have been noticed about Montreal, on the shores of Lake
Champlain, in Maine, and also in Sweden and Russia; and what is most
important, they are not everywhere at the same absolute level above
the surface of the ocean, shewing that both the subsidence and the
subsequent upheaval which has again brought them above the level of
the sea, have been unequal; and that we should therefore be very
cautious in our inferences respecting both the continental
circumstances under which the ancient glaciers were formed, and also
the extent of the sea afterward, as compared with its present limits.
The contrast between the unstratified drift and the subsequently
stratified deposits is so great, that they rest everywhere
unconformably upon each other, shewing distinctly the difference of
the agency under which they were accumulated. This unconformable
superposition of marine drift upon glacial drift is so beautifully
shewn at the above-mentioned locality near Cambridge (see diagram, p.
114.) In this case the action of tides in the accumulation of the
stratified materials is plainly seen.
The various heights at which these stratified deposits occur, above
the level of the sea, shew plainly, that since their accumulation the
main land has been lifted above the ocean at different rates in
different parts of the country; and it would be a most important
investigation to have their absolute level, in order more fully to
ascertain the last changes which our continents have undergone.
From the above mentioned facts, it must be at once obvious that the
various kinds of loose materials all over the northern hemisphere,
have been accumulated, not only under different circumstances, but
during long-continued subsequent distinct periods, and that great
changes have taken place since their deposition, before the present
state of things was fully established.
To the first period,--the ice period, as I have called it,--belong all
the phenomena connected with the transportation of erratic boulders,
the polishing, scratching, and furrowing of the rocks, and the
accumulation of unstratified, scratched, and loamy drift. During that
period the mainland seems to have been, to some extent at least,
higher above the level of the sea than now; as we observe, on the
shores of Great Britain, Norway, and Sweden, as well as on the eastern
shores of North America, the polished surfaces dipping under the level
of the ocean, which encroaches everywhere upon the erratics proper,
effaces the polished surfaces, and remodels the glacial drift. During
these periods, large terrestrial animals lived upon both continents,
the fossil remains of which are found in the drift of Siberia, as well
as of this continent. A fossil elephant, recently discovered in
Vermont, adds to the resemblance, already pointed out, between the
northern drift of Europe and that of North America; for fossils of
that genus are now known to occur upon the northern-most point of the
western extremity of North America, in New England, in Northern
Europe, as well as all over Siberia.
To the second period we would refer the stratified deposits resting
upon drift, which indicate, that during their deposition the northern
continent had again extensively subsided under the surface of the
ocean.
During this period, animals, identical with those which occur in the
northern seas, spread widely over parts of the globe which are now
again above the level of the ocean. But, as this last elevation seems
to have been gradual, and is even still going on in our day, there is
no possibility of tracing more precisely, at least for the present,
the limit between that epoch and the present state of things. Their
continuity seems almost demonstrated by the identity of fossil-shells
found in these stratified deposits, with those now living along the
present shores of the same continent, and by the fact, that changes in
the relative level between sea and mainland are still going on in our
day.
Indications of such relative changes between the level of the waters
and the land are also observed about Lake Superior. And here they
assume a very peculiar character, as the level of the lake itself, in
its relation to its shores, is extensively changed.[48]
Footnote 48: An interesting account of the natural terraces
around Lake Superior is given at p. 413-416 of "Lake Superior."
_Description of the Marine Telescope._
By JOHN ADIE, F.R.S.E., F.R.S.S.A. Communicated by the Author.
The instrument which has been popularly named the Water, or Marine
Telescope, from the power given by its use to see into the water,
consists of a tube of metal or wood, of a convenient length, to enable
a person looking over the gunnel of a boat to rest the head on the one
end, while the other is below the surface of the water; the upper end
is so formed, that the head may rest on it, both eyes seeing freely
into the tube. Into the lower end is fixed (water-tight) a plate of
glass, which, when used, is to be kept under the surface of the water.
[Illustration: sketch of marine telescope]
A very convenient size for the instrument represented in the above
figure, is to make the length AC, 3 feet, and the mouth A, where the
face is applied, of an irregular oval form, that both eyes may see
freely into the tube, with an indentation on one side, that the nose
may breathe freely, not throwing the moisture of the breath into the
tube. B is a round plate of glass, 8 inches diameter, over which is
the rim or edge C; this rim is best formed of lead, ¼ of an inch
thick, and 3 inches deep; the weight of the lead serves to sink the
tube a little into the water. Holes must be provided at the junction
of B to C, for the purpose of allowing the air to escape, and bring
the water into contact with the glass; on each side there is a handle
for holding the instrument. This size and form is very much that of
the instrument brought from Norway by John Mitchell, Esq., Belgian
Consul, of Mayville, with the improvement for excluding the breath,
and allowing the water to get into contact with the glass, which was
not provided for in that instrument.