America. From the extreme north down to the latitude of New York the
shores are obviously those of a partially-submerged region. They are
of the same type as the coasts of north-western Europe. We have every
reason to believe also that the depression of Greenland and north-east
America, from which these lands have only partially recovered,
dates back to a comparatively recent period. The fiords and inlets,
like those of Europe, are merely half-drowned land-valleys, and the
continental shelf is crossed by deep hollows which are evidently
only the seaward continuations of well-marked terrestrial features.
Such, for example, is the case with the valleys of the Hudson and
the St. Lawrence, the submerged portions of which can be followed
out to the edge of the continental plateau, which is notched by them
at depths of 474 and 622 fathoms respectively. There is, in short, a
broad resemblance between the coasts of the entire Arctic and North
Atlantic regions down to the latitudes already mentioned. Everywhere
they are irregular and fringed with islands in less or greater
abundance--highly-denuded and deeply-incised plateaux being penetrated
by fiords, while low-lying and undulating lands that shelve gently
seaward are invaded by shallow bays and inlets. Comparing the American
with the opposite European coasts one cannot help being struck with
certain other resemblances. Thus Hudson Bay at once suggests the
Baltic, and the Gulf of Mexico, with the Caribbean Sea, recalls the
Mediterranean. But the geological structure of the coast-lands of
Greenland and North America betrays a much closer resemblance between
these and the opposite shores of Europe than appears on a glance at
the map. There is something more than a mere superficial similarity.
In eastern North America and Greenland, just as in western Europe,
no grand mountain uplifts have taken place for a prodigious time.
The latest great upheavals, which were accompanied by much folding
and flexing of strata, are those of the Appalachian chain and of
the coastal ranges extending through New England, Nova Scotia, and
Newfoundland, all of which are of Palæozoic age. Considerable crustal
movements affected the American coast-lines in Mesozoic times, and
during these uplifts the strata suffered fracture and displacement,
but were subjected to comparatively little folding. Again, along
the maritime borders of north-east America, as in the corresponding
coast-lines of Europe, igneous action, more or less abundant in
Palæozoic and early Mesozoic times, has since been quiescent. From
the mouth of the Hudson to the Straits of Florida the coast-lines are
composed of Tertiary and Quaternary deposits. This shows that the land
has continued down to recent times to gain upon the sea--a result
brought about partly by quiet crustal movements, but to a large extent
by sedimentation, aided, on the coasts of Florida, by the action of
reef-building corals.
Although volcanic action has long ceased on the American sea-board,
we note that in Greenland, as in the west of Scotland and north
of Ireland, there is abundant evidence of volcanic activity at
so late a period as the Tertiary. It would appear that the great
plateau-basalts of those regions, and of Iceland and the Faröe Islands,
were contemporaneous, and were possibly connected with an important
crustal movement. It has long been suggested that at a very early
geological period Europe and North America may have been united. The
great thickness attained by the Palæozoic rocks in the eastern areas
of the latter implies the existence of a wide land-surface from which
ancient sediments were derived. That old land must have extended
beyond the existing coast-line, but how far we cannot tell. Similarly
in north-west Europe, during early Palæozoic times, the land probably
stretched further into the Atlantic than at present. But whether,
as some think, an actual land-connection subsisted between the two
continents it is impossible to say. Some such connection was formerly
supposed necessary to account for life common to the Palæozoic strata
of both continents, and which, as they were probably denizens of
comparatively shallow water, could only have crossed from one area
to another along a shore-line. It is obvious, indeed, that if the
oceanic troughs in those early days were of an abysmal character, a
belt of shallow water would be required to explain the geographical
distribution of cosmopolitan marine life-forms. But if it be true that
subsidence of the crust has been going on through all geological time,
and that the land-areas have nothwithstanding continued to extend over
the continental plateau, then it follows that the oceanic trough must
be deeper now than it was in Palæozoic times. There are, moreover,
certain geological facts which seem hardly explicable on the assumption
that the seas of past ages attained abysmal depths over any extensive
areas. The Palæozoic strata which enter so largely into the framework
of our lands have much the same appearance all the world over, and
were accumulated for the most part in comparatively shallow water.
A petrographical description of the Palæozoic mechanical sediments
of Europe would serve almost equally well for those of America, of
Asia, or of Australia. Take in connection with this the fact that
Palæozoic faunas had a very much wider range than those of Mesozoic and
later ages, and were characterised above all by the presence of many
cosmopolitan species, and we can hardly resist the conclusion that it
was the comparative shallowness of the ancient seas that favoured that
wide dispersal of species, and enabled currents to distribute sediments
the same in kind over such vast regions. As the oceanic area deepened
and contracted, and the land-surface increased, marine faunas were
gradually restricted in their range, and the cosmopolitan marine forms
diminished in numbers, while sediments, gathering in separate regions,
became more and more differentiated. For these and other reasons which
need not be entered upon here, I see no necessity for supposing that a
Palæozoic Atlantis connected Europe with North America. The broad ridge
upon which the Faröe Islands and Iceland are founded seems to pertain
as truly to the oceanic depression as the long Dolphin Ridge of the
South Atlantic. The trend of the continental plateau in high latitudes
is shown, as I think, by the general direction of the coast-lines of
north-western Europe and east Greenland, the continental shelf being
submerged in those regions for a few hundred fathoms only. How the
Icelandic ridge came into existence, and what its age may be, we can
only conjecture. It may be a wrinkle as old as the oceanic trough
which it traverses, or its origin may date back to a much more recent
period. We may conceive it to be an area which has subsided more slowly
than the floor of the ocean to the north and south; or, on the other
hand, it may be a belt of positive elevation. Perhaps the latter is
the more probable supposition, for it seems very unlikely that crustal
disturbances, resulting in axial and regional uplifts, should have been
confined to the continental plateau only. Be that as it may, there is
little doubt that land-connection did obtain between Greenland and
Europe in the Cainozoic times along this Icelandic ridge, for relics
of the same Tertiary flora are found in Scotland, the Faröe Islands,
Iceland, and Greenland. The deposits in which these plant-remains
occur are associated with great sheets of volcanic rocks, which in
the Faröe Islands and Iceland reach a thickness of many thousand feet.
Of the same age are the massive basalts of Jan Mayen, Spitzbergen,
Franz-Joseph Land, and Greenland. These lavas seem seldom to have
issued from isolated foci in the manner of modern eruptions, but rather
to have welled up along the lines of rectilineal fissures. From the
analogy of similar phenomena in other parts of the world it might be
inferred that the volcanic action of these northern regions may have
been connected with a movement of elevation, and that the Icelandic
ridge, if it did not come into existence during the Tertiary period,
was at all events greatly upheaved at that time. It would seem most
likely, in short, that the volcanic action in question was connected
mainly with crustal movements in the oceanic trough. Similar phenomena,
as is well known, are met with further south in the trough of the
Atlantic. Thus the volcanic Azores rise like Iceland from the surface
of a broad ridge which is separated from the continental plateau by
wide and deep depressions. And so again, from the back of the great
Dolphin Ridge, spring the volcanic islets of St. Paul's, Ascension, and
Tristan d'Acunha.
I have treated of the Icelandic bank at some length for the purpose of
showing that its volcanic phenomena do not really form an exception to
the rule that such eruptions ceased after Palæozoic or early Mesozoic
times to disturb the Atlantic coast-lines of Europe and North America.
As the bank in question extends between Greenland and the British
Islands, it was only natural that both those regions should be affected
by its movements. But its history pertains essentially to that of the
Atlantic trough; and it seems to show us how transmeridional movements
of the crust, accompanied by vast discharges of igneous rock, may come
in time to form land-connections between what are now widely-separated
areas.
Let us next turn our attention to the coast-lines of the Gulf of
Mexico and the Caribbean Sea. These enclosed seas have frequently been
compared to the Mediterranean, and the resemblance is self-evident.
Indeed, it is so close that one may say the Mexican-Caribbean Sea
and the Mediterranean are rather homologous than simply analogous.
The latter, as we have seen, occupies a primitive depression, and
formerly covered a much wider area. It extended at one time over
much of southern Europe and northern Africa, and appears to have
had full communication across Asia Minor with the Indian Ocean, and
with the Arctic Ocean athwart the low-lying tracts of north-western
Asia. Similarly, it would seem, the Mexican-Caribbean Sea is the
remaining portion of an ancient inland-sea which formerly stretched
north through the heart of North America to the Arctic Ocean. Like
its European parallel, it has been diminished by sedimentation and
crustal movements. It resembles the latter also in the greatness and
irregularity of its depths, and in the evidence which its islands
supply of volcanic action as well as of very considerable crustal
movements within recent geological times. Along the whole northern
borders of the Gulf of Mexico the coast-lands, like those on the
Atlantic sea-board of the Southern States, are composed of Tertiary
and recent accumulations, and the same is the case with Yucatan;
while similar young formations are met with on the borders of the
Caribbean Sea and in the Antilles. The Bahamas and the Windward
Islands mark out for us the margin of the continental plateau, which
here falls away abruptly to profound depths. One feels assured that
this portion of the plateau has been ridged up to its present level
at no distant geological date. But notwithstanding all the evidence
of recent extensive crustal movements in this region, it is obvious
that the Mexican-Caribbean depression, however much it may have been
subsequently modified, is of primitive origin.[DP]
[DP] Professor Suess thinks it is probable that the Caribbean Sea and
the Mediterranean are portions of one and the same primitive depression
which traversed the Atlantic area in early Cretaceous times. He further
suggests that it may have been through the gradual widening of the
central Mediterranean that the Atlantic in later times came into
existence.
Before we leave the coast-lands of North America, I would again point
out their leading geological features. In a word, then, they are
composed for the most part of Archæan and Palæozoic rocks; no great
linear or axial uplifts marked by much flexure of strata have taken
place in those regions since Palæozoic times; while igneous action
virtually ceased about the close of the Palæozoic or the commencement
of the Mesozoic period. It is not before we reach the shores of the
Southern States and the coast-lands of the Mexican-Caribbean Sea that
we encounter notable accumulations of Mesozoic, Tertiary, and younger
age. These occur in approximately horizontal positions round the Gulf
of Mexico; but in the Sierra Nevada of northern Colombia and the
Cordilleras of Venezuela the Tertiary strata enter into the formation
of true mountains of elevation. Thus the Mexican-Caribbean depression,
like that of the Mediterranean, is characterised not only by its
irregular depths and its volcanic phenomena, but by the propinquity
of recent mountains of upheaval, which bear the same relation to the
Caribbean Sea as the mountains of north Africa do to the Mediterranean.
We may now compare the Atlantic coasts of South America with those
of Africa. The former coincide in general direction with the edge
of the continental plateau, to which they closely approach between
Cape St. Roque and Cape Frio. In the north-east, between Cape Paria,
opposite Trinidad, and Cape St. Roque, the continental shelf attains
a considerably greater breadth, while south of Cape Frio it gradually
widens until, in the extreme south, it runs out towards the east in the
form of a narrow ridge, upon the top of which rise the Falkland Islands
and south Georgia. Excluding from consideration for the present all
recent alluvial and Tertiary deposits, we may say that the coast-lands
from Venezuela down to the south of Brazil are composed principally
of Archæan rocks; the eastern borders of the continent further south
being formed of Quaternary and Tertiary accumulations. So far as we
know, igneous rocks are of rare occurrence on the Atlantic sea-board.
Palæozoic strata approach the coast-lands at various points between
the mouths of the Amazons and La Plata, and these, with the underlying
and surrounding Archæan rocks, are more or less folded and disturbed,
while the younger strata of Mesozoic and Cainozoic age (occupying
wide regions in the basin of the Amazons, and here and there fringing
the sea-coast) occur in approximately horizontal positions. It would
appear, therefore, that no great axial uplifts have taken place in
those regions since Palæozoic times. The crustal movements of later
ages were regional rather than axial; the younger rocks are not flexed
and mashed together, and their elevation (negative or positive) does
not seem to have been accompanied by conspicuous volcanic action.
The varying width of the continental shelf is due to several causes.
The Orinoco, the Amazons, and other rivers descending to the north-east
coast, carry enormous quantities of sediment, much of which comes to
rest on the submerged slopes of the continental plateau, so that the
continental shelf tends to extend seawards. The same process takes
place on the south-east coast, where the Rio de la Plata discharges
its muddy waters. South of latitude 40° S., however, another cause
has come into play. From the mouth of the Rio Negro to the terminal
point of the continent the whole character of the coast betokens a
geologically recent emergence, accompanied and followed by considerable
marine erosion. So that in this region the continental shelf increases
in width by the retreat of the coast-line, while in the north-east it
gains by advancing seawards. It is to be noted, however, that even
there, in places where the shores are formed of alluvia, the sea tends
to encroach upon the land.
The Atlantic coast of Africa resembles that of South America in certain
respects, but it also offers some important contrasts. As the northern
coasts of Venezuela and Colombia must be considered in relation rather
to the Caribbean depression than to the Atlantic, so the African
sea-board between Cape Spartel and Cape Nun pertains structurally
to the Mediterranean region. From the southern limits of Morocco to
Cape Colony the coastal heights are composed chiefly of Archæan and
Palæozoic rocks, the low shore-lands showing here and there strata of
Mesozoic and Tertiary age together with still more recent deposits.
The existing coast-lines everywhere advance close to the edge of
the continental plateau, so that the submarine shelf is relatively
narrower than that of eastern South America. The African coast is still
further distinguished from that of South America by the presence of
several groups of volcanic islands--Fernando Po and others in the Gulf
of Guinea, and Cape Verde and Canary Islands. The last-named group,
however, notwithstanding its geographical position, is probably related
rather to the Mediterranean depression than to the Atlantic trough.
The geological structure of the African coast-lands shows that the
earliest to come into existence were those that extend between Cape
Nun and the Cape of Good Hope. The coastal ranges of that section are
much denuded, for they are of very great antiquity, having been ridged
up in Palæozoic times. The later uplifts (negative or positive) of
the same region were not attended by tilting and folding of strata,
for the Mesozoic and Tertiary deposits, like those of South America,
lie in comparatively horizontal positions. Between Cape Nun and Cape
Spartel the rocks of the maritime tracts range in age from Palæozoic to
Cainozoic, and have been traced across Morocco into Algeria and Tunis.
They all belong to the Mediterranean region, and were deposited at a
time when the southern shores of that inland sea extended from a point
opposite the Canary Islands along what is now the southern margin of
Morocco, Algeria, and Tunis. Towards the close of the Tertiary period
the final upheaval of the Atlas took place, and the Mediterranean,
retreating northwards, became an almost land-locked sea.
I need hardly stop to point out how the African coast-lines have been
modified by marine erosion and the accumulation of sediment upon the
continental shelf. The extreme regularity of the coasts is due partly
to the fact that the land is nearly co-extensive with the continental
plateau, but it also results in large measure from the extreme
antiquity of the land itself. This has allowed of the cutting-back of
headlands and the rilling up of bays and inlets, a process which has
been going on between Morocco and Cape Colony with probably little
interruption for a very prolonged period of time. We may note also the
effect of the heavy rains of the equatorial region in washing down
detritus to the shores, and in this way protecting the land to some
extent from the erosive action of the sea.
What now, let us ask, are the outstanding features of the coast-lines
of the Atlantic Ocean? We have seen that along the margins of each of
the bordering continents the last series of great mountain-uplifts
took place in Palæozoic times. This is true alike for North and South
America, for Europe and Africa. Later movements which have added to the
extent of land were not marked by the extreme folding of strata which
attended the early upheavals. The Mesozoic and Cainozoic rocks, which
now and again form the shore-lands, occur in more or less undisturbed
condition. The only great linear uplifts or true mountains of elevation
which have come into existence in western Europe and northern Africa
since the Palæozoic period trend approximately at right angles to the
direction of the Atlantic trough, and are obviously related to the
primitive depression of the Mediterranean. The Pyrenees and the Atlas,
therefore, although their latest elevation took place in Tertiary
times, form no exceptions to the rule that the extreme flexing and
folding of strata which is so conspicuous a feature in the geological
structure of the Atlantic sea-board dates back to the Palæozoic era.
And the same holds true of North and South America. There all the
coastal ranges of highly flexed and folded strata are of Palæozoic
age. The Cordilleras of Venezuela are no doubt a Tertiary uplift, but
they are as obviously related to the Caribbean depression as the Atlas
ranges are to that of the Mediterranean. Again, we note that volcanic
activity along the borders of the Atlantic was much less pronounced
during the Mesozoic period than it appears to have been in the earlier
ages. Indeed, if we except the great Tertiary basalt-flows of the
Icelandic ridge and the Arctic regions, we may say that volcanic action
almost ceased after the Palæozoic era to manifest itself upon the
Atlantic coast-lands of North America and Europe. But while volcanic
action has died out upon the Atlantic margins of both continents, it
has continued during a prolonged geological period within the area of
the Mediterranean depression. And in like manner the corresponding
depression between North and South America has been the scene of
volcanic disturbances from Mesozoic down to recent times. Along the
African coasts the only displays of recent volcanic action that
appertain to the continental margin are those of the Gulf of Guinea
and the Cape de Verde Islands. The Canary Islands and Madeira may come
under the same category, but, as we have seen, they appear to stand in
relationship to the Mediterranean depression and the Tertiary uplift
of North Africa. Of Iceland and the Azores I have already spoken, and
of Ascension and the other volcanic islets of the South Atlantic it is
needless to say that they are related to wrinkles in the trough of the
ocean, and therefore have no immediate connection with the continental
plateau.
Thus in the geographical development of the Atlantic coast-lines we may
note the following stages:--_First_, in Palæozoic times the formation
of great mountain-uplifts, frequently accompanied by volcanic action.
_Second_, a prolonged stage of comparative coastal tranquillity, during
which the maritime ranges referred to were subject to such excessive
erosion that they were planed down to low levels, and in certain areas
even submerged. _Third_, renewed elevation (negative or positive)
whereby considerable portions of the much-denuded Archæan and Palæozoic
rocks, now largely covered by younger deposits, were converted into
high-lands. During this stage not much rock-folding took place, nor
were any true mountains of elevation formed parallel to the Atlantic
margins. It was otherwise, however, in the Mediterranean and Caribbean
depressions, where coastal movements resulted in the formation of
enormous linear uplifts. Moreover, volcanic action is now and has for
a long time been more characteristic of these depressions than of the
Atlantic coast-lands.
I must now ask you to take a comprehensive glance at the coast-lines of
the Pacific Ocean. In some important respects these offer a striking
contrast to those we have been considering. Time will not allow me
to enter into detailed description, and I must therefore confine
attention to certain salient features. Examining first the shores
of the Americas, we find that there are two well-marked regions of
fiords and fringing islands--namely, the coasts of Alaska and British
Columbia, and of South America from 40° S.L. to Cape Horn. Although
these regions may be now extending seawards in places, it is obvious
that they have recently been subject to submergence. When the fiords of
Alaska and British Columbia existed as land-valleys it is probable that
a broad land-connection obtained between North America and Asia. The
whole Pacific coast is margined by mountain-ranges, which in elevation
and boldness far exceed those of the Atlantic sea-board. The rocks
entering into their formation range in age from Archæan and Palæozoic
down to Cainozoic, and they are almost everywhere highly disturbed and
flexed. It is not necessary, even if it were possible, to consider the
geological history of all those uplifted masses. It is enough for my
purpose to note the fact that the coastal ranges of North America and
the principal chain of the Andes were all elevated in Tertiary times.
It may be remarked further that, from the Mesozoic period down to the
present, the Pacific borders of America have been the scene of volcanic
activity far in excess of what has been experienced on the Atlantic
sea-board.