_THE HISTORY OF THE NORTH ATLANTIC._
I had the pleasure of being present at the meeting of the British
Association at Birmingham, in 1865: a meeting attended by an unusually
large number of eminent geologists, under the presidency of my friend
Phillips. I had the further pleasure of being his successor at the
meeting in the same place, in 1886; and the subject of this chapter
is that to which I directed the attention of the Association in my
Presidential address. I fear it is a feeble and imperfect utterance
compared with that which might have been given forth by any of the
great men present in 1865, and who have since left us, could they have
spoken with the added knowledge of the intervening twenty years.
The geological history of the Atlantic appeared to be a suitable
subject for a trans-Atlantic president, and to a Society which had
vindicated its claim to be British in the widest sense by holding a
meeting in Canada, while it was also meditating a visit to Australia--a
visit not yet accomplished, but in which it may now meet with a worthy
daughter in the Australian Association formed since the meeting of
1886. The subject is also one carrying our thoughts very far back in
geological time, and connecting itself with some of the latest and most
important discussions and discoveries in the science of the earth,
furnishing, indeed, too many salient points to be profitably occupied
in a single chapter.
If we imagine an observer contemplating the earth from a convenient
distance in space, and scrutinizing its features as it rolls
before him, we may suppose him to be struck with the fact that
eleven-sixteenths of its surface are covered with water, and that the
land is so unequally distributed that from one point of view he would
see a hemisphere almost exclusively oceanic, while nearly the whole of
the dry land is gathered in the opposite hemisphere. He might observe
that large portions of the great oceanic areas of the Pacific and
Antarctic Oceans are dotted with islands--like a shallow pool with
stones rising above its surface--as if the general depth were small
in comparison with the area. Other portions of these oceans he might
infer, from the colour of the water and the absence of islands, cover
deep depressions in the earth's surface. He might also notice that a
mass or belt of land surrounds each pole, and that the northern ring
sends off to the southward three vast tongues of land and of mountain
chains, terminating respectively in South America, South Africa, and
Australia, towards which feebler and insular processes are given off
by the antarctic continental mass. This, as some geographers have
observed,[21] gives a rudely three-ribbed aspect to the earth, though
two of the ribs are crowded together, and form the Eurasian mass or
double continent, while the third is isolated in the single continent
of America. He might also observe that the northern girdle is cut
across, so that the Atlantic opens by a wide space into the Arctic Sea,
while the Pacific is contracted toward the north, but confluent with
the Antarctic Ocean. The Atlantic is also relatively deeper and less
cumbered with islands than the Pacific, which has the highest ridges
near its shores, constituting what some visitors to the Pacific coast
of America have not inaptly called the "back of the world," while the
wider slopes face the narrower ocean. The Pacific and Atlantic, though
both depressions or flattenings of the earth, are, as we shall find,
different in age, character, and conditions; and the Atlantic, though
the smaller, is the older, and, from the geological point of view, in
some respects, the more important of the two; while, by virtue of its
lower borders and gentler slope, it is, though the smaller basin, the
recipient of the greater rivers, and of a proportionately great amount
of the drainage of the land.[22]
[21] Dana, "Manual of Geology," introductory part. Green, "Vestiges of
a Molten Globe," has summed up these facts.
[22] Mr. Mellard Reade, in two Presidential addresses before the
Geological Society of Liverpool, has illustrated this point and its
geological consequences.
If our imaginary observer had the means of knowing anything of the
rock formations of the continents, he would notice that those bounding
the North Atlantic are, in general, of great age some belonging to
the Laurentian system. On the other hand, he would see that many of
the mountain ranges along the Pacific are comparatively new, and that
modern igneous action occurs in connection with them. Thus he might see
in the Atlantic, though comparatively narrow, a more ancient feature of
the earth's surface; while the Pacific belongs to more modern times.
But he would note, in connection with this, that the oldest rocks of
the great continental masses are mostly toward their northern ends;
and that the borders of the northern ring of land, and certain ridges
extending southward from it, constitute the most ancient and permanent
elevations of the earth's crust, though now greatly surpassed by
mountains of more recent age nearer the equator, so that the continents
of the northern hemisphere seem to have grown progressively from north
to south.
If the attention of our observer were directed to more modern
processes, he might notice that while the antarctic continent freely
discharges its burden of ice to the ocean north of it, the arctic ice
has fewer outlets, and that it mainly discharges itself through the
North Atlantic, where also the great mass of Greenland stands as a huge
condenser and cooler, unexampled elsewhere in the world, throwing
every spring an immense quantity of ice into the North Atlantic, and
more especially into its western part. On the other hand, he might
learn from the driftage of weed and the colour of the water, that the
present great continuous extension and form of the American continent
tend to throw northward a powerful branch of the equatorial current,
which, revolving around the North Atlantic, counteracts the great flow
of ice which otherwise would condemn it to a perpetual winter.
Further, such an observer would not fail to notice that the ridges
which lie along the edges of the oceans and the ebullitions of igneous
matter which proceed, or have proceeded from them, are consequences
of the settling downward of the great oceanic depressions, a settling
ever intensified by their receiving more and more of deposit on their
surfaces; and that this squeezing upward of the borders of these
depressions into folds has been followed or alternated with elevations
and depressions without any such folding, and proceeding from other
causes. On the whole, it would be apparent that these actions are more
vigorous now at the margins of the Pacific area, while the Atlantic is
backed by very old foldings, or by plains and slopes from which it has,
so to speak, dried away without any internal movement. Thus it would
appear that the Pacific is the great centre of earth-movement, while
the Atlantic trench is the more potent regulator of temperature, and
the ocean most likely to be severely affected in this respect by small
changes of its neighbouring land. Last of all, an observer, such as I
have supposed, would see that the oceans are the producers of moisture
and the conveyors of heat to the northern regions of the world, and
that in this respect and in the immense condensation and delivery of
ice at its north end, the Atlantic is by far the more active, though
the smaller of the two.
So much could be learned by an extra-mundane observer; but unless he
had also enjoyed opportunities of studying the rocks of the earth in
detail and close at hand, or had been favoured by some mundane friend
with a perusal of "Lyell's Elements," or "Dana's Manual," he would not
be able to appreciate as we can the changes which the Atlantic has seen
in geological time, and in which it has been a main factor. Nor could
he learn from such superficial observation certain secrets of the deep
sea, which have been unveiled by the sounding lead, the inequalities of
the ocean basin, its few profound depths, like inverted mountains or
table-lands, its vast nearly flat abyssmal floor, and the sudden rise
of this to the hundred fathom line, forming a terrace or shelf around
the sides of the continents. These features, roughly represented in the
map prefixed, he would be unable to perceive.
Before leaving this broad survey, we may make one further remark. An
observer, looking at the earth from without, would notice that the
margins of the Atlantic and the main lines of direction of its mountain
chains are north-east and south-west, and north-west and south-east, as
if some early causes had determined the occurrence of elevations along
great circles of the earth's surface tangent to the polar circles.
We are invited by the preceding general glance at the surface of the
earth to ask certain questions respecting the Atlantic, (1) What has
at first determined its position and form? (2) What changes has it
experienced in the lapse of geological time? (3) What relations have
these changes borne to the development of life on the land and in the
water? (4) What is its probable future?
Before attempting to answer these questions, which I shall not take
up formally in succession, but rather in connection with each other,
it is necessary to state, as briefly as possible, certain general
conclusions respecting the interior of the earth. It is popularly
supposed that we know nothing of this beyond a superficial crust
perhaps averaging 50,000 to 100,000 feet in thickness. It is true we
have no means of exploration in the earth's interior, but the conjoined
labours of physicists have now proceeded sufficiently far to throw
much inferential light on the subject, and to enable us to make some
general affirmations with certainty; and these it is the more necessary
to state distinctly, since they are often treated as mere subjects of
speculation and fruitless discussion.
(1) Since the dawn of geological science, it has been evident that the
crust on which we live must be supported on a plastic or partially
liquid mass of heated rock, approximately uniform in quality under
the whole of its area. This is a legitimate conclusion from the
wide distribution of volcanic phenomena, and from the fact that the
ejections of volcanoes, while locally of various kinds, are similar in
every part of the world. It led to the old idea of a fluid interior of
the earth, but this seems now generally abandoned, and this interior
heated and plastic layer is regarded as merely an under-crust, resting
on a solid nucleus.[23]
[23] I do not propose to express any definite opinion as to this
question, as either conclusion will satisfy the demands of geology. It
would seem, however, that astronomers now admit a slight periodical
deformation of the crust. See Lord Kelvin's Anniversary Address to
Royal Society, 1892.
(2) We have reason to believe, as the result of astronomical
investigations,[24] that, notwithstanding the plasticity or liquidity
of the under-crust, the mass of the earth--its nucleus as we may call
it--is practically solid and of great density and hardness. Thus we
have the apparent paradox of a solid yet fluid earth; solid in its
astronomical relations, liquid or plastic for the purposes of volcanic
action and superficial movements.
[24] Hopkins, Mallet, Lord Kelvin, and Prof. G. H. Darwin maintain
the solidity and rigidity of the earth on astronomical grounds; but
different conclusions have been reached by Fisher, Hennesey, Delaunay,
and Airy. In America, Hunt, Barnard and Crosby, Button, Le Conte and
Wadsworth have discussed these questions. Bonney has suggested that a
mass may be slowly mobile under long-continued pressure, while rigid
with reference to more sudden movements.
(3) The plastic sub-crust is not in a state of dry igneous fusion, but
in that condition of aqueo-igneous or hydrothermic fusion which arises
from the action of heat on moist substances, and which may either
be regarded as a fusion or as a species of solution at a very high
temperature. This we learn from the phenomena of volcanic action, and
from the composition of the volcanic and plutonic rocks, as well as
from such chemical experiments as those of Daubrée, and of Tilden, and
Shenstone.[25] It follows that water or steam, as well as rocky matter,
may be ejected from the under-crust.
[25] _Phil. Trans._, 1884. Also Crosby in _Proc. Boston Soc. Nat.
Hist._, 1883.
(4) The interior sub-crust is not perfectly homogeneous, but may be
roughly divided into two layers or magmas, as they have been called;
an upper, highly silicious or acidic, of low specific gravity and
light-coloured, and corresponding to such kinds of plutonic and
volcanic rocks as granite and trachyte; and a lower, less silicious
or more basic, more dense, and more highly charged with iron, and
corresponding to such igneous rocks as the dolerites, basalts, and
kindred lavas. It is interesting here to note that this conclusion,
elaborated by Durocher and Von Waltershausen, and usually connected
with their names, appears to have been first announced by John
Phillips, in his "Geological Manual," and as a mere common sense
deduction from the observed phenomena of volcanic action and the
probable results of the gradual cooling of the earth. It receives
striking confirmation from the observed succession of acidic and basic
volcanic rocks of all geological periods and in all localities. It
would even seem, from recent spectroscopic investigations of Lockyer,
that there is evidence of a similar succession of magmas in the
heavenly bodies, and the discovery by Nordenskiöld of native iron in
Greenland basalts, affords a probability that the inner magma is in
part metallic, and possibly, that vast masses of unoxidised metals
exist in the central portion of the earth.
(5) Where rents or fissures form in the upper crust, the material of
the lower crust is forced upward by the pressure of the less supported
portions of the former, giving rise to volcanic phenomena either of
an explosive or quiet character, as may be determined by contact with
water. The underlying material may also be carried to the surface by
the agency of heated water, producing those quiet discharges which Hunt
has named crenitic. It is to be observed here that explosive volcanic
phenomena, and the formation of cones, are, as Prestwich has well
remarked, characteristic of an old and thickened crust; quiet ejection
from fissures and hydro-thermal action may have been more common in
earlier periods and with a thinner over-crust This is an important
consideration with reference to those earlier ages referred to in
chapter second.
(6) The contraction of the earth's interior by cooling and by the
emission of material from below the over-crust, has caused this crust
to press downward, and therefore laterally, and so to effect great
bends, folds, and plications; and these, modified subsequently by
surface denudation, and the piling of sediments on portions of the
crust, constitute mountain chains and continental plateaus. As Hall
long ago pointed out,[26] such lines of folding have been produced more
especially where thick sediments had been laid down on the sea-bottom,
and where, in consequence, internal expansion of the crust had occurred
from heating below. Thus we have here another apparent paradox, namely,
that the elevations of the earth's crust occur in the places where the
greatest burden of detritus has been laid down upon it, and where,
consequently, the crust has been softened and depressed. We must
beware, in this connection, of exaggerated notions of the extent of
contraction and of crumpling required to form mountains. Bonney has
well shown, in lectures delivered at the London Institution, that an
amount of contraction, almost inappreciable in comparison with the
diameter of the earth, would be sufficient; and that, as the greatest
mountain chains are less than 1/600th of the earth's radius in height,
they would, on an artificial globe a foot in diameter, be no more
important than the slight inequalities that might result from the
paper gores overlapping each other at the edges. This thinness of the
crushed crust agrees with the deductions of physical science as to
the shallowness of the superficial layer of compression in a cooling
globe. It is perhaps not more than five miles in thickness. A singular
proof of this is seen by the extension of straight cracks filled with
volcanic rock in the Laurentian districts of Canada.[27] The beds of
gneiss and associated rocks are folded and crumpled in a most complex
manner, yet they are crossed by these faults, as a crack in a board
may tear a sheet of paper or a thin veneer glued on it. We thus see
that the crumpled Laurentian crust was very thin, while the uncrushed
sub-crust determined the line of fracture.
[26] Hall (American Association Address, 1857, subsequently
republished, with additions, as "Contributions to the Geological
History of the American Continent"), Mallet, Rogers, Dana, La Conte,
etc.
[27] As, for instance, the great dyke running nearly in a straight line
from near St. Jerome along the Ottawa to Templeton, on the Ottawa, and
beyond, a distance of more than a hundred miles.
(7) The crushing and sliding of the over-crust implied in these
movements raise some serious questions of a physical character. One
of these relates to the rapidity or slowness of such movements,
and the consequent degree of intensity of the heat developed, as a
possible cause of metamorphism of rocks. Another has reference to
the possibility of changes in the equilibrium of the earth itself,
as resulting from local collapse and ridging. These questions in
connection with the present dissociation of the axis of rotation
from the magnetic poles, and with changes of climate, have attracted
some attention,[28] and probably deserve further consideration on the
part of physicists. In so far as geological evidence is concerned, it
would seem that the general association of crumpling with metamorphism
indicates a certain rapidity in the process of mountain-making, and
consequent development of heat; and the arrangement of the older rocks
around the Arctic basin forbids us from assuming any extensive movement
of the axis of rotation, though it does not exclude changes to a
limited extent.
[28] See recent papers of Oldham and Fisher, in _Geological Magazine_,
and _Philosophical Magazine_, July, 1886. Also Péroche, "Revol.
Polaires." Paris, 1886.
(8) It appears from the above that mountains and continental elevations
may be of three kinds, (_a_) They may consist of material thrown out
of volcanic rents, like earth out of a mole burrow. Mountains like
Vesuvius and Ætna are of this kind. (_b_) They may be parts of wide
ridges or chains variously cut and modified by rains and rivers. The
Lebanon and the Catskill Mountains are cases in point, (_c_) They may
be lines of crumpling by lateral pressure. The greatest mountains,
like the Cordillera, the Alps, and the Appalachians are of this kind,
and such mountains may represent lateral pressure occurring at various
times, and whose results have been greatly modified subsequently.
I wish to formulate these principles as distinctly as possible, and as
the result of all the long series of observations, calculations, and
discussions since the time of Werner and Hutton, and in which a vast
number of able physicists and naturalists have borne a part, because
they may be considered as certain deductions from our actual knowledge,
and because they lie at the foundation of a rational physical geology.
We may roughly popularise these deductions by comparing the earth to a
drupe or stone-fruit, such as a plum or peach somewhat dried up. It
has a large and intensely hard stone and kernel, a thin pulp made up
of two layers, an inner, more dense and dark-coloured, and an outer,
less dense and lighter-coloured. These constitute the under-crust. On
the outside it has a thin membrane or over-crust. In the process of
drying it has slightly shrunk, so as to produce ridges and hollows
of the outer crust, and this outer crust has cracked in some places,
allowing portions of the pulp to ooze out--in some of them its lower
dark substance, in others, its upper and lighter material. The analogy
extends no farther, for there is nothing in our withered fruit to
represent the oceans occupying the lower parts of the surface, or the
deposits which they have laid down.
Here a most important feature demands attention. The rain, the streams,
and the sea are constantly cutting down the land and depositing it in
the bed of the waters. Thus weight is taken from the land, and added
to the sea bed. Geological facts, such as the great thickness of the
coal measures, in which we find thousands of feet of sediment, all of
which must have been deposited in shallow water, and the accumulation
of hundreds of feet of superficial material in deltas at the mouth
of great rivers, show that the crust of the earth is so mobile as to
yield downward to every pressure, however slight.[29] It may do this
slowly and gradually, or by jumps from time to time; and this yielding
necessarily tends to squeeze up the edges of the depressed portions
into ridges, and to cause lateral movement and ejection of volcanic
matter at intervals.
[29] Starkie Gardiner, _Nature_, December, 1889.
Keeping in view these general conclusions, let us now turn to their
bearing on the origin and history of the North Atlantic.
Though the Atlantic is a deep ocean, its basin does not constitute so
much a depression of the crust of the earth as a flattening of it, and
this, as recent soundings have shown, with a slight ridge or elevation
along its middle, and banks or terraces fringing the edges, so that its
form is not so much that of a basin as that of a shallow elongated
plate with its middle a little raised. Its true margins are composed
of portions of the over-crust folded, overlapped and crushed, as if by
lateral pressure emanating from the sea itself. We cannot, for example,
look at a geological map of America without perceiving that the
Appalachian ridges, which intervene between the Atlantic and the St.
Lawrence valley, have been driven bodily back by a force acting from
the east, and that they have resisted this pressure only where, as in
the Gulf of St. Lawrence and the Catskill region of New York, they have
been protected by outlying masses of very old rocks, as, for example,
by that of the island of Newfoundland and that of the Adirondack
Mountains. The admirable work begun by my friend and fellow-student,
Professor James Nicol, followed up by Professor Lapworth, and now,
after long controversy, fully confirmed by the recent observations
of the Geological Survey of Scotland, has shown the most intense
action of the same kind on the east side of the ocean in the Scottish
highlands; and the more widely distributed Eozoic and other old rocks
of Scandinavia may be appealed to in further evidence of this.[30]
[30] Address to Geological Section, Brit. Assoc., by Prof. Judd,
Aberdeen Meeting, 1885. According to Rogers, the crumpling of the
Appalachians has reduced a breadth of 158 miles to about 60. Geikie,
Address, Geological Society, 1891-2.
If we now inquire as to the cause of the Atlantic depression, we must
go back to the time when the areas occupied by the Atlantic and its
bounding coasts were parts of the shoreless sea in which the earliest
gneisses or stratified granites of the Laurentian age were being laid
down in vastly extended beds. These ancient crystalline rocks have been
the subject of much discussion and controversy, to which reference has
been made in a previous chapter.
It will be observed, in regard to these theories, that they do not
suppose that the old gneiss is an ordinary sediment, but that all
regard it as formed in exceptional circumstances, these circumstances
being the absence of land and of subaërial decay of rock, and the
presence wholly or principally of the material of the upper surface
of the recently hardened crust. This being granted, the question
arises, Ought we not to combine the several theories as to the origin
of gneiss, and to believe that the cooling crust has hardened in
successive layers from without inward; that at the same time fissures
were locally discharging igneous matter to the surface; that matter
held in suspension in the ocean and matter held in solution by heated
waters rising from beneath the outer crust were mingling their
materials in the deposits of the primitive ocean?[31] It would seem
that the combination of all these agencies may safely be evoked as
causes of the pre-Atlantic deposits. This is the eclectic position I
have maintained in a previous chapter, and which I hold to be in every
way the most probable.
[31] Hunt, _Transactions Royal Society of Canada_, 1885.
Let us suppose, then, the floor of old ocean covered with a flat
pavement of gneiss, or of that material which is now gneiss, the next
question is, How and when did this original bed become converted into
sea and land? Here we have some things certain, others most debatable.
That the cooling mass, especially if it was sending out volumes of
softened rocky material, either in the form of volcanic ejections or
in that of matter dissolved in heated water, and piling this on the
surface, must soon become too small for its shell, is apparent; but
when and where would the collapse, crushing and wrinkling inevitable
from this cause begin? The date is indicated by the lines of old
mountain chains which traverse the Laurentian districts; but the reason
why is less apparent. The more or less unequal cooling, hardening
and conductive power of the outer crust we may readily assume. The
driftage unequally of water-borne detritus to the south-west by the
bottom currents of the sea is another cause, and, as we shall soon see,
most effective. Still another is the greater cooling and hardening of
the crust in the polar regions, and the tendency to collapse of the
equatorial protuberance from the slackening of the earth's rotation.
Besides these, the internal tides of the earth's substance at the
times of solstice would exert an oblique pulling force on the crust,
which might tend to crack it along diagonal lines. From whichever of
these causes, or the combination of the whole, we know that, within
the Laurentian time, folded portions of the earth's crust began to
rise above the general surface, in broad belts running from north-east
to south-west, and from north-west to south-east, where the older
mountains of Eastern America and Western Europe now stand, and that
the subsidence of the oceanic areas, allowed by this crumpling of
the crust, permitted other areas on both sides of the Atlantic to
form limited table-lands. This was the commencement of a process
repeated again and again in subsequent times, and which began in the
middle Laurentian, when for the first time we find beds of quartzite,
limestone, and iron ore, and graphite beds, indicating that there was
already land and water, and that the sea, and perhaps the land, swarmed
with forms of animal and plant life, unknown, for the most part, now.
Independently of the questions as to the animal nature of Eozoon, I
hold that we know, as certainly as we can know anything inferentially,
the existence of these primitive forms of life. If I were to conjecture
what were these early forms of plant and animal life, still unknown to
us by actual specimens, I would suppose that, just as in the Palæozoic,
the acrogens culminated in gigantic and complex forest trees, so in the
Laurentian, the algæ, the lichens, and the mosses grew to dimensions
and assumed complexity of structure unexampled in later times, and
that, in the sea, the humbler forms of Protozoa and Sea Mosses were the
dominant types, but in gigantic and complex forms. The land of this
period was probably limited, for the most part, to high latitudes, and
its aspect, though more rugged and abrupt, and of greater elevation,
must have been of that character which we still see in the Laurentian
hills. The distribution of this ancient land is indicated by the long
lines of old Laurentian rock extending from the Labrador coast and
the north shore of the St. Lawrence, and along the eastern slopes
of the Appalachians in America, and the like rocks of the Hebrides,
the Western Highlands, and the Scandinavian mountains. A small but
interesting remnant is that in the Malvern Hills, so well described by
Holl. It will be well to note here, and to fix on our minds, that these
ancient ridges of Eastern America and Western Europe have been greatly
denuded and wasted since Laurentian times, and that it is along their
eastern sides that the greatest sedimentary accumulations have been
deposited.
From this time dates the introduction of that dominance of existing
causes which forms the basis of uniformitarianism in geology, and which
had to go on with various and great modifications of detail, through
the successive stages of the geological history, till the land and
water of the northern hemisphere attained to their present complex
structure.
So soon as we have a circumpolar belt or patches of Eozoic[32] land and
ridges running southward from it, we enter on new and more complicated
methods of growth of the continents and seas. Portions of the oldest
crystalline rocks, raised out of the protecting water, were now eroded
by atmospheric agents, and especially by the carbonic acid, then
existing in the atmosphere perhaps more abundantly than at present,
under whose influence the hardest of the gneissic rocks gradually
decay. The arctic lands were subjected, in addition, to the powerful
mechanical force of frost and thaw. Thus every shower of rain and
every swollen stream would carry into the sea the products of the
waste of land, sorting them into fine clays and coarser sands; and the
cold currents which cling to the ocean bottom, now determined in their
courses, not merely by the earth's rotation, but also by the lines of
folding on both sides of the Atlantic, would carry south-westward, and
pile up in marginal banks of great thickness the _débris_ produced
from the rapid waste of the land already existing in the Arctic
regions. The Atlantic, opening widely to the north, and having large
rivers pouring into it, was, especially, the ocean characterised, as
time advanced, by the prevalence of these phenomena. Thus, throughout
the geological history it has happened that, while the middle of the
Atlantic has received merely organic deposits of shells of foraminifera
and similar organisms, and this probably only to a small amount, its
margins have had piled upon them beds of detritus of immense thickness.
Professor Hall, of Albany, was the first geologist who pointed out
the vast cosmic importance of these deposits, and that the mountains
of both sides of the Atlantic owe their origin to these great lines
of deposition, along with the fact, afterwards more fully insisted on
by Rogers, that the portions of the crust which received these masses
of _débris_ became thereby weighted down and softened, and were more
liable than other parts to lateral crushing.
[32] Or Archæan, or pre-Cambrian, if these terms are preferred.
Thus, in the later Eozoic and early Palæozoic times, which succeeded
the first foldings of the oldest Laurentian, great ridges were thrown
up, along the edges of which were beds of limestone, and on their
summits and sides, thick masses of ejected igneous rocks. In the bed
of the central Atlantic there are no such accumulations. It must have
been a flat, or slightly ridged, plate of the ancient gneiss, hard and
resisting, though perhaps with a few cracks, through which igneous
matter welled up, as in Iceland and the Azores in more modern times.
In this condition of things we have causes tending to perpetuate and
extend the distinctions of ocean and continent, mountain and plain,
already begun; and of these we may more especially note the continued
subsidence of the areas of greatest marine deposition. This has long
attracted attention, and affords very convincing evidence of the
connection of sedimentary deposit as a cause with the subsidence of the
crust.[33]
[33] Dutton in _Report of U.S. Geological Survey_, 1891. From facts
stated in this report and in my "Acadian Geology," it is apparent
that in the Western States and in the coal fields of Nova Scotia,
shallow-water deposits have been laid down, up to thicknesses of 10,000
to 20,000 feet in connection with continuous subsidence. See also a
paper by Ricketts in the _Geol. Mag._, 1883.
We are indebted to a French physicist, M. Faye, for an important
suggestion on this subject. It is that the sediment accumulated
along the shores of the ocean presented an obstacle to radiation,
and consequently to cooling of the crust, while the ocean floor,
unprotected and unweighted, and constantly bathed with currents of cold
water having great power of convection of heat, would be more rapidly
cooled, and so would become thicker and stronger. This suggestion
is complementary to the theory of Professor Hall, that the areas of
greatest deposit on the margins of the ocean are necessarily those of
greatest folding and consequent elevation. We have thus a hard, thick,
resisting ocean bottom, which, as it settles down toward the interior,
under the influence of gravity, squeezes upwards and folds and plicates
all the soft sediments deposited on its edges. The Atlantic area is
almost an unbroken cake of this kind. The Pacific area has cracked in
many places, allowing the interior fluid matter to exude in volcanic
ejections.
It may be said that all this supposes a permanent continuance of
the ocean basins, whereas many geologists postulate a mid-Atlantic
continent to give the thick masses of detritus found in the older
formations both in Eastern America and Western Europe, and which thin
off in proceeding into the interior of both continents. I prefer, as
already stated, to consider these belts of sediment as the deposits
of northern currents, and derived from arctic land, and that, like
the great banks off the American coast at the present day, which are
being built up by the present arctic current, they had little to do
with any direct drainage from the adjacent shore. We need not deny,
however, that such ridges of land as existed along the Atlantic margins
were contributing their quota of river-borne material, just as on a
still greater scale the Amazon and Mississippi are doing now, and
this especially on the sides toward the present continental plateaus,
though the greater part must have been derived from the wide tracts
of Laurentian land within the Arctic Circle, or near to it. It is
further obvious that the ordinary reasoning respecting the necessity
of continental areas in the present ocean basins would actually
oblige us to suppose that the whole of the oceans and continents
had repeatedly changed places. This consideration opposes enormous
physical difficulties to any theory of alternations of the oceanic and
continental areas, except locally at their margins.
But the permanence of the Atlantic depression does not exclude the idea
of successive submergences of the continental plateaus and marginal
slopes, alternating with periods of elevation, when the ocean retreated
from the continents and contracted its limits. In this respect the
Atlantic of to-day is much smaller than it was in those times when it
spread widely over the continental plains and slopes, and much larger
than it has been in times of continental elevation. This leads us to
the further consideration that, while the ocean beds have been sinking,
other areas have been better supported, and constitute the continental
plateaus; and that it has been at or near the junctions of these
sinking and rising areas that the thickest deposits of detritus, the
most extensive foldings, and the greatest ejections of volcanic matter
have occurred. There has thus been a permanence of the position of
the continents and oceans throughout geological time, but with many
oscillations of these areas, producing submergences and emergences of
the land. In this way we can reconcile the vast vicissitudes of the
continental areas in different geological periods with that continuity
of development from north to south, and from the interiors to the
margins, which is so marked a feature. We have, for this reason, to
formulate another apparent geological paradox, namely, that while, in
one sense, the continental and oceanic areas are permanent, in another,
they have been in continual movement. Nor does this view exclude
extension of the continental borders or of chains of islands beyond
their present limits, at certain periods; and indeed, the general
principle already stated, that subsidence of the ocean bed has produced
elevation of the land, implies in earlier periods a shallower ocean and
many possibilities as to volcanic islands, and low continental margins
creeping out into the sea; while it is also to be noted that there are,
as already stated, bordering shelves, constituting shallows in the
ocean, which at certain periods have emerged as land.
We are thus compelled, as already stated, to believe in the
contemporaneous existence in all geological periods, except perhaps
the earliest of them, of the three distinct conditions of areas on the
surface of the earth, defined in chapter second oceanic areas of deep
sea, continental plateaus and marginal shelves, and lines of plication
and folding.
In the successive geological periods the continental plateaus, when
submerged, owing to their vast extent of warm and shallow sea, have
been the great theatres of the development of marine life and of
the deposition of organic limestones, and when elevated, they have
furnished the abodes of the noblest land faunas and floras. The
mountain belts, especially in the north, have been the refuge and
stronghold of land life in periods of submergence; and the deep
ocean basins have been the perennial abodes of pelagic and abyssal
creatures and the refuge of multitudes of other marine animals and
plants in times of continental elevation. These general facts are full
of importance with reference to the question of the succession of
formations and of life in the geological history of the earth.
So much space has been occupied with these general views, that it would
be impossible to trace the history of the Atlantic in detail through
the ages of the Palæozoic, Mesozoic, and Tertiary. We may, however,
shortly glance at the changes of the three kinds of surface already
referred to. The bed of the ocean seems to have remained, on the whole,
abyssal; but there were probably periods when those shallow reaches of
the Atlantic which stretch across its most northern portion, and partly
separate it from the Arctic basin, presented connecting coasts or
continuous chains of islands sufficient to permit animals and plants to
pass over.[34] At certain periods also there were, not unlikely, groups
of volcanic islands, like the Azores, in the temperate or tropical
Atlantic. More especially might this be the case in that early time
when it was more like the present Pacific; and the line of the great
volcanic belt of the Mediterranean, the mid-Atlantic banks, the Azores
and the West India Islands point to the possibility of such partial
connections. These were stepping stones, so to speak, over which land
organisms might cross, and some of these may be connected with the
fabulous or pre-historic Atlantis.
[34] It would seem, from Geikie's description of the Faroe Islands,
that they may be a remnant of such connecting land, dating from the
Cretaceous or Eocene period.
In the Palæozoic period, the distinctions already referred to, into
continental plateaus, mountain ridges, and ocean depths, were first
developed, and we find, already, great masses of sediment accumulating
on the seaward sides of the old Laurentian ridges, and internal
deposits thinning away from these ridges over the submerged continental
areas, and presenting dissimilar conditions of sedimentation. It would
seem also that, as Hicks has argued for Europe, and Logan and Hall
for America, this Cambrian age was one of slow subsidence of the land
previously elevated, accompanied with or caused by thick deposits of
detritus along the borders of the subsiding shore, which was probably
covered with the decomposing rock arising from long ages of subaërial
waste.
In the coal formation age its characteristic swampy flats stretched
in some places far into the shallower parts of the ocean.[35] In the
Permian, the great plicated mountain margins were fully developed on
both sides of the Atlantic. In the Jurassic, the American continent
probably extended farther to the sea than at present. In the Wealden
age there was much land to the west and north of Great Britain,
and Professor Bonney has directed attention to the evidence of the
existence of this land as far back as the Trias, while Mr. Starkie
Gardiner has insisted on connecting links to the southward, as
evidenced by fossil plants. So late as the Post-glacial, or early human
period, large tracts, now submerged, formed portions of the continents.
On the other hand, the interior plains of America and Europe were often
submerged. Such submergences are indicated by the great limestones
of the Palæozoic, by the chalk and its representative beds in the
Cretaceous, by the Nummulitic formation in the Eocene, and lastly,
by the great Pleistocene submergence, one of the most remarkable of
all, one in which nearly the whole northern hemisphere participated,
and which was probably separated from the present time by only a few
thousands of years.[36] These submergences and elevations were not
always alike on the two sides of the Atlantic. The Salina period of the
Silurian, for example, and the Jurassic, show continental elevation in
America not shared by Europe. The great subsidences of the Cretaceous
and the Eocene were proportionally deeper and wider on the eastern
continent, and this and the direction of the land being from north to
south, cause more ancient forms of life to survive in America. These
elevations and submergences of the plateaus alternated with the periods
of mountain-making plication, which was going on at intervals, at the
close of the Eozoic, at the beginning of the Cambrian, at the close of
the Siluro-Cambrian, in the Permian, and in Europe and Western America
in the Tertiary. The series of changes, however, affecting all these
areas was of a highly complex character in detail.[37]
[35] I have shown the evidence of this in the remnants of Carboniferous
districts once more extensive on the Atlantic coast of Nova Scotia and
Cape Breton ("Acadian Geology").
[36] The recent surveys of the Falls of Niagara coincide with a great
many evidences to which I have elsewhere referred in proving that the
Pleistocene submergence of America and Europe came to an end not more
than ten thousand years ago, and was itself not of very great duration.
Thus in Pleistocene times the land must have been submerged and
re-elevated in a very rapid manner.
[37] "Acadian Geology."
We may also note a fact which I have long ago insisted on,[38] the
regular pulsation of the continental areas, giving us alternations
in each great system of deep-sea and shallow-water beds, so that
the successive groups of formations may be divided into triplets of
shallow-water, deep-water, and shallow-water strata, alternating in
each period. This law of succession applies more particularly to
the formations of the continental plateaus, rather than to those of
the ocean margins, and it shows that, intervening between the great
movements of plication there were subsidences of those plateaus, or
elevations of the sea bottom, which allowed the waters to spread
themselves over all the inland spaces between the great folded mountain
ranges of the Atlantic borders.
[38] "Acadian Geology."
In referring to the ocean basins we should bear in mind that there are
three of these in the northern hemisphere the Arctic, the Pacific, and
the Atlantic. De Ranee has ably summed up the known facts as to Arctic
geology in a series of articles in _Nature_, from which it appears
that this area presents from without inwards a succession of older and
newer formations from the Eozoic to the Tertiary, and that its extent
must have been greater in former periods than at present, while it
must have enjoyed a comparatively warm climate from the Cambrian to
the Pleistocene period. The relations of its deposits and fossils are
closer with those of the Atlantic than with those of the Pacific, as
might be anticipated from its wider opening into the former. Blandford
has recently remarked on the correspondence of the marginal deposits
around the Pacific and Indian oceans,[39] and Dr. Dawson informs me
that this is equally marked in comparison with the west coast of
America, but these marginal areas have not yet gained much on the
ocean. In the North Atlantic, on the other hand, there is a wide belt
of comparatively modern rocks on both sides, more especially toward the
south and on the American side; but while there appears to be a perfect
correspondence on both sides of the Atlantic, and around the Pacific
respectively, there seems to be less parallelism between the deposits
and forms of life of the two oceans, as compared with each other, and
less correspondence in forms of life, especially in modern times.
Still, in the earlier geological ages, as might have been anticipated
from the imperfect development of the continents, the same forms of
life characterise the whole ocean from Australia to Arctic America, and
indicate a grand unity of Pacific and Atlantic life not equalled in
later times,[40] and which speaks of true contemporaneity rather than
of what has been termed homotaxis or mere likeness of orders.
[39] _Journal of Geological Society_, May, 1886. Blandford's statements
respecting the mechanical deposits of the close of the Palæozoic in
the Indian Ocean, whether these are glacial or not, would seem to show
a correspondence with the Permian conglomerates and earth-movements
of the Atlantic area; but since that time the Atlantic has enjoyed
comparative repose. The Pacific seems to have reproduced the conditions
of the Carboniferous in the Cretaceous age, and seems to have been less
affected by the great changes of the Pleistocene.
[40] Daintree and Etheridge, "Queensland Geology," _Journal Geological
Society_, August, 1872; R. Etheridge, Junior, "Australian Fossils,"
_Trans. Phys. Soc. Edin._, 1880.
We may pause here for a moment to notice some of the effects of
Atlantic growth on modern geography. It has given us rugged and broken
shores, composed of old rocks in the north, and newer formations and
softer features toward the south. It has given us marginal mountain
ridges and internal plateaus on both sides of the sea. It has produced
certain curious and by no means accidental correspondences of the
eastern and western sides. Thus the solid basis on which the British
Islands stand may be compared with Newfoundland and Labrador, the
English Channel with the Gulf of St. Lawrence, the Bay of Biscay with
the Bay of Maine, Spain with the projection of the American land at
Cape Hatteras, the Mediterranean with the Gulf of Mexico. The special
conditions of deposition and plication necessary to these results, and
their bearing on the character and productions of the Atlantic basin,
would require a volume for their detailed elucidation.
Thus far our discussion has been limited almost entirely to physical
causes and effects. If we now turn to the life history of the Atlantic,
we are met at the threshold with the question of climate, not as a
thing fixed and immutable, but as changing from age to age in harmony
with geographical mutations, and producing long cosmic summers and
winters of alternate warmth and refrigeration.
We can scarcely doubt that the close connection of the Atlantic and
Arctic oceans is one factor in those remarkable vicissitudes of climate
experienced by the former, and in which the Pacific area has also
shared in connection with the Antarctic Sea. No geological facts are
indeed at first sight more strange and inexplicable than the changes
of climate in the Atlantic area, even in comparatively modern periods.
We know that in the early Tertiary temperate conditions reigned as
far north as the middle of Greenland, and that in the Pleistocene the
Arctic cold advanced until an almost perennial winter prevailed half
way to the equator. It is no wonder that nearly every cause available
in the heavens and the earth has been invoked to account for these
astounding facts. I shall, I trust, be excused if, neglecting most of
these theoretical views, I venture to invite attention, in connection
with this question, chiefly to the old Lyellian doctrine of the
modification of climate by geographical changes. Let us, at least,
consider how much these are able to account for.
The ocean is a great equalizer of extremes of temperature. It does this
by its great capacity for heat, and by its cooling and heating power
when passing from the solid into the liquid and gaseous states, and the
reverse. It also acts by its mobility, its currents serving to convey
heat to great distances, or to cool the air by the movement of cold
icy waters. The land, on the other hand, cools or warms rapidly, and
can transmit its influence to a distance only by the winds, and the
influence so transmitted is rather in the nature of a disturbing than
of an equalizing cause. It follows that any change in the distribution
of land and water must affect climate, more especially if it changes
the character or course of the ocean currents.
Turning to the Atlantic, in this connection we perceive that its
present condition is peculiar and exceptional. On the one hand it is
widely open to the Arctic Sea and the influence of its cold currents,
and on the other it is supplied with a heating apparatus of enormous
power to give a special elevation of temperature, more particularly to
its eastern coasts. The great equatorial current running across from
Africa is on its northern side embayed in the Gulf of Mexico, as in a
great cauldron, and pouring through the mouth of this in the Bahama
channel, forms the gulf stream, which, widening out like a fan, forms
a vast expanse of warm water, from which the prevailing westerly winds
of the North Atlantic waft a constant supply of heated moist air to
the western coasts of Europe, giving them a much more warm and uniform
climate than that which prevails in similar latitudes in Eastern
America, where the cold Arctic currents hug the shore, and bring down
ice from Baffin's Bay. Now all this might be differently arranged. We
shall find that there were times, when the Isthmus of Panama being
broken through, there was no Gulf Stream, and Norway and England
were reduced to the conditions of Greenland and Labrador, and when
refrigeration was still further increased by subsidence of northern
lands affording freer sweep to the Arctic currents. On the other hand,
there were times when the Gulf of Mexico extended much farther north
than at present, and formed an additional surface of warm water to heat
all the interior of America, as well as the Atlantic. Geographical
changes of these kinds, have probably given us the glacial period in
very recent times, and at an earlier era those warm climates which
permitted temperate vegetation to flourish as far north as Greenland.
These are, however, great topics, which must form the subject of other
chapters.
I am old enough to remember the sensation caused by the delightful
revelations of Edward Forbes respecting the zones of animal life in
the sea, and the vast insight which they gave into the significance of
the work on minute organisms previously done by Ehrenberg, Lonsdale
and Williamson, and into the meaning of fossil remains. A little later
the soundings for the Atlantic cable revealed the chalky foraminiferal
ooze of the abyssal ocean. Still more recently, the wealth of facts
disclosed by the _Challenger_ voyage, which naturalists have scarcely
yet had time to digest, have opened up to us new worlds of deep-sea
life.
The bed of the deep Atlantic is covered, for the most part, by a mud
or ooze, largely made up of the _débris_ of foraminifera and other
minute organisms mixed with fine clay. In the North Atlantic the
Norwegian naturalists call this the Biloculina mud. Farther south, the
_Challenger_ naturalists speak of it as Globigerina ooze. In point of
fact it contains different species of foraminiferal shells, Globigerina
and Orbulina being in some localities dominant, and in others, other
species; and these changes are more apparent in the shallower portions
of the ocean.
On the other hand, there are means for disseminating coarse material
over parts of the ocean beds. There are, in the line of the Arctic
current, on the American coast, great sand banks, and off the coast of
Norway, sand constitutes a considerable part of the bottom material.
Soundings and dredgings off Great Britain, and also off the American
coast, have shown that fragments of stone referable to Arctic lands
are abundantly strewn over the bottom, along certain lines, and the
Antarctic continent, otherwise almost unknown, makes its presence
felt to the dredge by the abundant masses of crystalline rock drifted
far from it to the north. These are not altogether new discoveries.
I had inferred, many years ago, from stones taken up by the hooks of
fishermen on the banks of Newfoundland, that rocky material from the
north is dropped on these banks by the heavy ice which drifts over them
every spring, that these are glaciated, and that after they fall to the
bottom sand is drifted over them with sufficient velocity to polish the
stones, and to erode the shelly coverings of Arctic animals attached
to them.[41] If, then, the Atlantic basin were upheaved into land, we
should see beds of sand, gravel and boulders with clay flats and layers
of marl and limestone. According to the _Challenger_ reports, in the
Antarctic seas S. of 64 there is blue mud, with fragments of rock, in
depths of 1,200 to 2,000 fathoms. The stones, some of them glaciated,
were granite, diorite, amphibolite, mica schist, gneiss and quartzite.
This deposit ceases and gives place to Globigerina ooze and red clay
at 46° to 47° S., but even farther north there is sometimes as much as
49 per cent, of crystalline sand. In the Labrador current a block of
syenite, weighing 400 lbs., was taken up from 1,340 fathoms, and in the
Arctic current, 100 miles from land, was a stony deposit, some stones
being glaciated. Among these were smoky quartz, quartzite, limestone,
dolomite, mica schist, and serpentine; also particles of monoclinic and
triclinic felspar, hornblende, augite, magnetite, mica and glauconite,
the latter, no doubt, formed in the sea bottom, the others drifted from
Eozoic and Palæozoic formations to the north.[42]
[41] "Notes on Post-Pliocene of Canada," 1872.
[42] _General Report, "Challenger" Expedition_.
A remarkable fact in this connection is that the great depths of
the sea are as impassable to the majority of marine animals as the
land itself. According to Murray, while twelve of the _Challenger's_
dredgings, taken in depths greater than 2,000 fathoms, gave 92 species,
mostly new to science, a similar number of dredgings in shallower water
near the land, give no less than 1,000 species. Hence arises another
apparent paradox relating to the distribution of organic beings. While
at first sight it might seem that the chances of wide distribution
are exceptionally great for marine species, this is not so. Except in
the case of those which enjoy a period of free locomotion when young,
or are floating and pelagic, the deep ocean sets bounds to their
migrations. On the other hand, the spores of cryptogamic plants may
be carried for vast distances by the wind, and the growth of volcanic
islands may effect connections which, though only temporary, may afford
opportunity for land animals and plants to pass over.
With reference to the transmission of living beings across the
Atlantic, we have before us the remarkable fact that from the Cambrian
age onwards there were, on the two sides of the ocean, many species
of invertebrate animals which were either identical or so closely
allied as to be possibly varietal forms, indicating probably the
shallowness of the ocean in these periods. In like manner, the early
plants of the Upper Silurian, Devonian, and Carboniferous present many
identical species; but this identity is less marked in more modern
times. Even in the latter, however, there are remarkable connections
between the floras of oceanic islands and the continents. Thus the
Bermudas, altogether recent islands, have been stocked by the agency
chiefly of the ocean currents and of birds, with nearly 150 species
of continental plants; and the facts collected by Helmsley as to
the present facilities of transmission, along with the evidence
afforded by older oceanic islands which have been receiving animal and
vegetable colonists for longer periods, go far to show that, time being
given, the sea actually affords facilities for the migration of the
inhabitants of the land, comparable with those of continuous continents.
In so far as plants are concerned, it is to be observed that the early
forests were largely composed of cryptogamous plants, and the spores of
these in modern times have proved capable of transmission from great
distances. In considering this, we cannot fail to conclude, that the
union of simple cryptogamous fructification with arboreal stems of
high complexity, so well illustrated by Dr. Williamson, had a direct
relation to the necessity for a rapid and wide distribution of these
ancient trees. It seems also certain that some spores, as, for example,
those of the Rhizocarps,[43] a type of vegetation abundant in the
Palæozoic, and certain kinds of seeds, as those named _Æthoetesta_
and _Pachytheca_, were fitted for flotation. Further, the periods of
Arctic warmth permitted the passage around the northern belt of many
temperate species of plants, just as now happens with the Arctic flora;
and when these were displaced by colder periods, they marched southward
along both sides of the sea on the mountain chains.
[43] See paper by the author on Palæozoic Rhizocarps, _Chicago Trans._,
1886.
The same remark applies to northern forms of marine invertebrates,
which are much more widely distributed in longitude than those farther
south. The late Mr. Gywn Jeffreys, in one of his latest communications
on this subject, stated that 54 per cent, of the shallow-water mollusks
of New England and Canada are also European, and of the deep-sea forms,
30 out of 35; these last, of course, enjoying greater facilities for
migration than those which have to travel slowly along the shallows of
the coast in order to cross the ocean and settle themselves on both
sides. Many of these animals, like the common mussel and sand clam,
are old settlers which came over in the Pleistocene period, or even
earlier. Others, like the common periwinkle, seem to have been slowly
extending themselves in modern times, perhaps even by the agency of
man. The older immigrants may possibly have taken advantage of lines of
coast now submerged, or of warm periods, when they could creep round
the Arctic shores. Mr. Herbert Carpenter and other naturalists employed
on the _Challenger_ collections have made similar statements respecting
other marine invertebrates, as, for instance, the Echinoderms, of
which the deep-sea crinoids present many common species, and my own
collections prove that many of the shallow-water forms are common.
Dall and Whiteaves[44] have shown that some mollusks and Echinoderms
are common even to the Atlantic and Pacific coasts of North America;
a remarkable fact, testifying at once to the fixity of these species
and to the manner in which they have been able to take advantage of
geographical changes. Some of the species of whelks common to the Gulf
of St. Lawrence and the Pacific are animals which have no special
locomotive powers, even when young, but they are northern forms not
proceeding far south, so that they may have passed through the Arctic
seas. In this connection it is well to remark that many species of
animals have powers of locomotion in youth which they lose when adult,
and that others may have special means of transit. I once found at
Gaspé a specimen of the Pacific species of Coronula, or whale-barnacle,
the _C. reginæ_ of Darwin, attached to a whale taken in the Gulf of
St. Lawrence, and which had possibly succeeded in making that passage
around the north of America which so many navigators have essayed in
vain.[45]
[44] Dall, _Report on Alaska_; Whiteaves, _Trans. R. S. C._
[45] I am informed, however, that the Coronula is found also in the
Biscayan whales.
But it is to be remarked that while many plants and marine
invertebrates are common to the two sides of the Atlantic, it is
different with land animals, and especially vertebrates. I do not know
that any palæozoic insects or land snails or millipedes of Europe and
America are specifically identical, and of the numerous species of
batrachians of the Carboniferous and reptiles of the Mesozoic, all seem
to be distinct on the two sides. The same appears to be the case with
the Tertiary mammals, until in the later stages of that great period we
find such genera as the horse, the camel, and the elephant appearing
on the two sides of the Atlantic; but even then the species seem
different, except in the case of a few northern forms.
Some of the longer-lived mollusks of the Atlantic furnish suggestions
which remarkably illustrate the biological aspect of these questions.
Our familiar friend the oyster is one of these. The first-known oysters
appear in the Carboniferous in Belgium and in the United States of
America. In the Carboniferous and Permian they are few and small, and
they do not culminate till the Cretaceous, in which there are no less
than ninety-one so-called species in America alone; but some of the
largest known species are found in the Eocene. The oyster, though an
inhabitant of shallow water, and very limitedly locomotive when young,
has survived all the changes since the Carboniferous age, and has
spread itself over the whole northern hemisphere,[46] though a warm
water rather than Arctic type.
[46] White, _Report U. S. Geol. Survey_, 1882-3.
I have collected fossil oysters in the Cretaceous clays of the coulées
of Western Canada, in the Lias shales of England, in the Eocene and
the Cretaceous beds of the Alps, of Egypt, of the Red Sea coast, of
Judea, and the heights of Lebanon. Everywhere and in all formations
they present forms which are so variable and yet so similar that one
might suppose all the so-called species to be mere varieties. Did the
oyster originate separately on the two sides of the Atlantic, or did it
cross over so promptly that its appearance seems to be identical on the
two sides? Are all the oysters of a common ancestry, or did the causes,
whatever they were, which introduced the oyster in the Carboniferous
act over again in later periods? Who can tell? This is one of the
cases where causation and development--the two scientific factors
which constitute the basis of what is called evolution--cannot easily
be isolated. I would recommend to those biologists who discuss these
questions to devote themselves to the oyster. This familiar mollusk has
successfully, pursued its course, and has overcome all its enemies,
from the flat-toothed selachians of the Carboniferous to the oyster
dredges of the present day, has varied almost indefinitely, and yet has
continued to be an oyster, unless, indeed, it may at certain portions
of its career have temporarily assumed the guise of a Gryphæa or an
Exogyra. The history of such an animal deserves to be traced with care,
and much curious information respecting it will be found in the report
which I have cited in the note.
But in these respects the oyster, is merely an example of many forms.
Similar considerations apply to all those Pliocene and Pleistocene
mollusks which are found in the raised sea bottoms of Norway and
Scotland, on the top of Moel Tryfaen, in Wales, and at similar great
heights on the hills of America, many of which can be traced back to
early Tertiary times, and can be found to have extended themselves
over all the seas of the northern hemisphere. They apply in like
manner to the ferns, the conifers, and the broad-leaved trees, many
of which we can now trace without specific change to the Eocene and
Cretaceous. They all show that the forms of living things are more
stable than the lands and seas in which they live. If we were to adopt
some of the modern ideas of evolution, we might cut the Gordian knot
by supposing that, as like causes produce like effects, these types
of life have originated more than once in geological time, and need
not be genetically connected with each other. But while evolutionists
repudiate such an application of their doctrine, however natural and
rational, it would seem that nature still more strongly repudiates it,
and will not allow us to assume more than one origin for one species.
Thus the great question of geographical distribution remains in all its
force; and, by still another of our geological paradoxes, mountains
become ephemeral things in comparison with the delicate herbage which
covers them, and seas are in their present extent but of yesterday,
when compared with the minute and feeble organisms that creep on their
sands or swim in their waters.
The question remains: Has the Atlantic achieved its destiny and
finished its course, or are there other changes in store for it in
the future? The earth's crust is now thicker and stronger than ever
before, and its great ribs of crushed and folded rock are more firm and
rigid than in any previous period. The stupendous volcanic phenomena
manifested in Mesozoic and early Tertiary times along the borders
of the Atlantic have apparently died out. These facts are in so far
guarantees of permanence. On the other hand, it is known that movements
of elevation, along with local depression, are in progress in the
Arctic regions, and a great weight of new sediment is being deposited
along the borders of the Atlantic, especially on its western side;
and this is not improbably connected with the earthquake shocks and
slight movements of depression which have occurred in North America.
It is possible that these slight and secular movements may go on
uninterruptedly, or with occasional paroxysmal disturbances, until
considerable changes are produced.
It is possible, on the other hand, that after the long period of
quiescence which has elapsed, there may be a new settlement of the
ocean bed, accompanied with foldings of the crust, especially on the
western side of the Atlantic, and possibly with renewed volcanic
activity on its eastern margin. In either case, a long time relatively
to our limited human chronology may intervene before the occurrence
of any marked change. On the whole, the experience of the past would
lead us to expect movements and eruptive discharges in the Pacific
rather than in the Atlantic area. It is therefore not unlikely that the
Atlantic may remain undisturbed, unless secondarily and indirectly,
until after the Pacific area shall have attained to a greater degree of
quiescence than at present. But this subject is one too much involved
in uncertainty to warrant us in following it farther.
In the meantime the Atlantic is to us a practically permanent ocean,
varying only in its tides, its currents, and its winds, which science
has already reduced to definite laws, so that we can use if we cannot
regulate them. It is ours to take advantage of this precious time of
quietude, and to extend the blessings of science and of our Christian
civilisation from shore to shore, until there shall be no more sea,
not in the sense of that final drying-up of old ocean to which some
physicists look forward, but in the higher sense of its ceasing to be
the emblem of unrest and disturbance, and the cause of isolation.
I must now close this chapter with a short statement of some general
truths which I have had in view in directing attention to the
geological development of the Atlantic. We cannot, I think, consider
the topics to which I have referred without perceiving that the history
of ocean and continent is an example of progressive design, quite as
much as that of living beings. Nor can we fail to see that, while
in some important directions we have penetrated the great secret of
nature, in reference to the general plan and structure of the earth
and its waters, and the changes through which they have passed, we
have still very much to learn, and perhaps quite as much to unlearn,
and that the future holds out to us and to our successors higher,
grander, and clearer conceptions than those to which we have yet
attained. The vastness and the might of ocean and the manner in which
it cherishes the feeblest and most fragile beings, alike speak to us of
Him who holds it in the hollow of His hand, and gave to it of old its
boundaries and its laws; but its teaching ascends to a higher tone when
we consider its origin and history, and the manner in which it has been
made to build up continents and mountain-chains, and, at the same time,
to nourish and sustain the teeming life of sea and land.
References:--Presidential Address to the British Association for the
Advancement of Science, Birmingham, 1886. "Geology of Nova Scotia,
New Brunswick, and Prince Edward Island." Fourth Edition, London,
1891.
_THE DAWN OF LIFE._
DEDICATED TO THE MEMORY OF
SIR WILLIAM E. LOGAN,
The unwearied Explorer of the Laurentian Rocks,
and the Founder
of the
Geological Survey of Canada.
What Are the Oldest Rocks, and where?--Conditions of their
Formation--Indications of Life--What its probable Nature
[Illustration: Nature-print of Eozoon, showing laminated, acervuline,
and fragmental portions.
This is printed from an electrotype taken from an etched slab of
Eozoon, and not touched with a graver except to remedy some accidental
flaws in the plate. The diagonal white line marks the course of a
calcite vein.]