contrast to those of the American borders. The latter, as we have
seen, are for the most part not far removed from the edge of the
continental plateau. The coasts of the mainland of Asia, on the other
hand, retire to a great distance, the true margin of the plateau being
marked out by that great chain of islands which extends from Kamchatka
south to the Philippines and New Guinea. The seas lying between those
islands and the mainland occupy depressions in the continental plateau.
Were that plateau to be lifted up for 6000 or 7000 feet the seas
referred to would be enclosed by continuous land, and all the principal
islands of the East Indian Archipelago--Sumatra, Java, Celebes, and New
Guinea, would become united to themselves as well as to Australia and
New Zealand. In short, it is the relatively depressed condition of the
continental plateau along the western borders of the Pacific basin that
causes the Asiatic coast-lines to differ so strikingly from those of
America.
From a geological point of view the differences are less striking than
the resemblances. It is true that we have as yet a very imperfect
knowledge of the geological structure of eastern Asia, but we know
enough to justify the conclusion that in its main features that region
does not differ essentially from western North America. During Mesozoic
and Cainozoic times the sea appears to have overflowed vast tracts of
Manchooria and China, and even to have penetrated into what is now the
great Desert of Gobi. Subsequent crustal movements revolutionised the
geography of all those regions. Great ranges of linear uplifts came
into existence, and in these the younger formations, together with
the foundations on which they rested, were squeezed into folds and
ridged up against the nuclei of Palæozoic and Archæan rocks which had
hitherto formed the only dry land. The latest of these grand upheavals
are of Tertiary age, and, like those of the Pacific slope of America,
they were accompanied by excessive volcanic action. The long chains of
islands that flank the shores of Asia we must look upon as a series of
partially submerged or partially emerged mountain-ranges, analogous
geographically to the coast-ranges of North and Central America, and
to the youngest Cordilleras of South America. The presence of numerous
active and recently extinct volcanoes, taken in connection with the
occurrence of many great depressions which furrow the floor of the sea
in the East Indian Archipelago, and the profound depths attained by the
Pacific trough along the borders of Japan and the Kurile and Aleutian
Islands--all indicate conditions of very considerable instability of
the lithosphere. We are not surprised, therefore, to meet with much
apparently conflicting evidence of elevation and depression in the
coast-lands of eastern Asia, where in some places the sea would seem
to be encroaching, while in other regions it is retreating. In all
earthquake-ridden and volcanic areas such irregular coastal changes
may be looked for. So extreme are the irregularities of the sea-floor
in the area lying between Australia, the Solomon Islands, the New
Hebrides, and New Zealand, and so great are the depths attained by
many of the depressions, that the margins of the continental plateau
are harder to trace here than anywhere else in the world. The bottom
of the oceanic trough throughout a large portion of the southern and
western Pacific is, in fact, traversed by many great mountain-ridges,
the summits of which approach the surface again and again to form the
numerous islets of Polynesia. But notwithstanding the considerable
depths that separate Australia from New Zealand there is geological
evidence to show that a land-connection formerly linked both to Asia.
The continental plateau, therefore, must be held to include New
Caledonia and New Zealand. Hence the volcanic islets of the Solomon and
New Hebrides groups are related to Australia in the same way as the
Liu-kiu, Japanese, and Kurile Islands are to Asia.
Having rapidly sketched the more prominent features of the Pacific
coast-lines, we are in a position to realise the remarkable contrast
they present to the coast-lines of the Atlantic. The highly-folded
strata of the Atlantic sea-board are the relics of great mountains of
upheaval, the origin of which cannot be assigned to a more recent date
than Palæozoic times. During subsequent crustal movements no mountains
of corrugated strata were uplifted along the Atlantic margins, the
Mesozoic and Cainozoic strata of the coastal regions showing little
or no disturbance. It is quite in keeping with all this that volcanic
action appears to have been most strongly manifested in Palæozoic
times. So many long ages have passed since the upheaval of the Archæan
and Palæozoic mountains of the Atlantic sea-board that these heights
have everywhere lost the character of true mountains of elevation.
Planed down to low levels, partially submerged and covered to some
extent by newer formations, they have in many places been again
converted into dry lands, forming plateaux--now sorely denuded and
cut up into mountains and valleys of erosion. Why the later movements
along the borders of the Atlantic basin should not have resulted in
the wholesale plication of the younger sedimentary rocks is a question
for geologists. It would seem as if the Atlantic margins had reached a
stage of comparative stability long before the grand Tertiary uplifts
of the Pacific borders had taken place; for, as we have seen, the
Mesozoic and the Cainozoic strata of the Atlantic coast-lands show
little or no trace of having been subjected to tangential thrusting
and crushing. Hence one cannot help suspecting that the retreat of
the sea during Mesozoic and Cainozoic ages may have been due rather
to subsidence of the oceanic trough and to sedimentation within the
continental area than to positive elevation of the land.
Over the Pacific trough, likewise, depression has probably been in
progress more or less continuously since Palæozoic times, and this
movement alone must have tended to withdraw the sea from the surface
of the continental plateau in Asia and America. But by far the most
important coastal changes in those regions have been brought about
by the crumpling-up of the plateau, and the formation of gigantic
mountains of upheaval along its margins. From remotest geological
periods down almost to the present, the land-area has been increased
from time to time by the doubling-up and consequent elevation of
coastal accumulations, and by the eruption of vast masses of volcanic
materials. It is this long-continued activity of the plutonic forces
within the Pacific area which has caused the coast-lands of that basin
to contrast so strongly with those of the Atlantic. The latter are
incomparably older than the former--the heights of the Atlantic borders
being mountains of denudation of vast geological antiquity, while the
coastal ranges of the Pacific slope are creations but of yesterday as
it were. It may well be that those Cordilleras and mountain-chains
reach a greater height than was ever attained by any Palæozoic uplifts
of the Atlantic borders. But the marked disparity in elevation between
the coast-lands of the Pacific and the Atlantic is due chiefly to a
profound difference in age. Had the Pacific coast-lands existed for as
long a period and suffered as much erosion as the ancient rocks of the
Atlantic sea-board, they would now have little elevation to boast of.
The coast-lines of the Indian Ocean are not, upon the whole, far
removed from the margin of the continental plateau. The elevation of
East Africa for 6000 feet would add only a narrow belt to the land.
This would still leave Madagascar an island, but there are geological
reasons for concluding that this island was at a far distant period
united to Africa, and it must therefore be considered as forming
a portion of the continental plateau. The great depths which now
separate it from the mainland are probably due to local subsidence,
connected with volcanic action in Madagascar itself and in the Comoro
Islands. The southern coasts of Asia, like those of East Africa,
approach the edge of the continental plateau, so that an elevation
of 6000 feet would make little addition to the land-area. With the
same amount of upheaval, however, the Malay Peninsula, Sumatra, Java,
and West Australia would become united, but without extending much
further seawards. Land-connection, as we know, existed in Mesozoic
times between Asia, Australia, and New Zealand, but the coast-lines
of that distant period must have differed considerably from those
that would appear were the regions in question to experience now a
general elevation. The Archæan and the Palæozoic rocks of the Malay
Peninsula and Sumatra are flanked on the side of the Indian Ocean
by great volcanic ridges, and by uplifts of Tertiary strata, which
continue along the line of the Nicobar and the Andaman Islands into
Burma. Thus the coast-lines of that section of the Indian Ocean exhibit
a geographical development similar to that of the Pacific sea-board.
Elsewhere, as in Hindustan, Arabia, and East Africa, the coast-lines
appear to have been determined chiefly by regional elevations of the
land or subsidence of the oceanic trough in Mesozoic and Cainozoic
times, accompanied by the out-welling of enormous floods of lava.
Seeing, then, that the Pacific and the Indian Oceans are pre-eminently
regions which, down to a recent date, have been subject to great
crustal movements and to excessive volcanic action, we may infer
that in the development of their coast-lines the sea played a very
subordinate part. The shores, indeed, are largely protected from marine
erosion by partially emerged volcanic ridges and by coral islands
and reefs, and to a considerable extent also by the sediment which
in tropical regions especially is swept down to the coast in great
abundance by rains and rivers. Moreover, as the geological structure of
these regions assures us, the land would appear seldom to have remained
sufficiently long at one level to permit of much destruction by waves
and tidal currents.
In fine, then, we arrive at the general conclusion that the coast-lines
of the globe are of very unequal age. Those of the Atlantic were
determined as far back as Palæozoic times by great mountain-up lifts
along the margin of the continental plateau. Since the close of that
period many crustal oscillations have taken place, but no grand
mountain-ranges have again been ridged up on the Atlantic sea-board.
Meanwhile the Palæozoic mountain-chains, as we have seen, have suffered
extensive denudation, have been planed down to sea-level, and even
submerged. Subsequently converted into land, wholly or partially as
the case may have been, they now present the appearance of plains
and plateaux of erosion, often deeply indented by the sea. No true
mountains of elevation are met with anywhere in the coast-lands of the
Atlantic, while volcanic action has well-nigh ceased. In short, the
Atlantic margins have reached a stage of comparative stability. The
trough itself, however, is traversed by at least two well-marked banks
of upheaval--the great meridional Dolphin Ridge, and the approximately
transmeridional Faröe-Icelandic belt--both of them bearing volcanic
islands.
But while all the coast-lands of the Atlantic proper attained relative
stability at an early period, those of the Mediterranean and Caribbean
depressions have up to recent times been the scenes of great crustal
disturbance. Gigantic mountain-chains were uplifted along their margins
at so late a period as the Tertiary, and their shores still witness
volcanic activity.
It is upon the margins and within the trough of the Pacific Ocean,
however, that subterranean action is now most remarkably developed.
The coast-lines of that great basin are everywhere formed of grand
uplifts and volcanic ranges, which, broadly speaking, are comparable
in age to those of the Mediterranean and Caribbean depressions. Along
the north-eastern margin of the Indian Ocean the coast-lines resemble
those of the Pacific, being of like recent age, and similarly marked
by the presence of numerous volcanoes. The northern and western
shores, however (as in Hindustan, Arabia, and East Africa), have
been determined rather by regional elevation or by subsidence of the
ocean-floor than by axial uplifts--the chief crustal disturbances
dating back to an earlier period than those of the East Indian
Archipelago. It is in keeping with this greater age of the western and
northern coast-lands of the Indian Ocean that volcanic action is now
less strongly manifested in their vicinity.
I have spoken of the comparative stability of the earth's crust within
the Atlantic area as being evidenced by the greater age of its coastal
ranges and the declining importance of its volcanic phenomena. This
relative stability is further shown by the fact that the Atlantic
sea-board is not much disturbed by earthquakes. This, of course, is
what might have been expected, for earthquakes are most characteristic
of volcanic regions and of those areas in which mountain-uplifts of
recent geological age occur. Hence the coast-lands of the Pacific and
the East Indies, the borders of the Caribbean Sea, the volcanic ridges
of the Atlantic basin, the lands of the Mediterranean, the Black Sea,
and the Aralo-Caspian depressions, the shores of the Red Sea, and vast
tracts of southern Asia, are the chief earthquake regions of the globe.
It may be noted, further, that shocks are not only most frequent but
most intense in the neighbourhood of the sea. They appear to originate
sometimes in the volcanic ridges and coastal ranges, sometimes under
the floor of the sea itself. Now earthquakes, volcanoes, and uplifts
are all expressions of the one great fundamental fact that the earth
is a cooling and contracting body, and they indicate the lines of
weakness along which the enormous pressures and strains induced by the
subsidence of the crust upon its nucleus find relief. We cannot tell
why the coast-lands of the Atlantic should have attained at so early
a period a stage of relative stability--why no axial uplifts should
have been developed along their margins since Palæozoic times. It may
be that relief has been found in the wrinkling-up of the floor of the
oceanic trough, and consequent formation of the Dolphin Ridge and other
great submarine foldings of the crust; and it is possible that the
growth of similar great ridges and wrinkles upon the bed of the Pacific
may in like manner relieve the coast-lands of that vast ocean, and
prevent the formation of younger uplifts along their borders.
I have already remarked that two kinds of elevatory movements of
the crust are recognised by geologists--namely, axial and regional
uplifts. Some, however, are beginning to doubt, with Professor
Suess, whether any vast regional uplifts are possible. Yet the view
that would attribute all such apparent elevations of the land to
subsidence of the crust under the great oceanic troughs is not without
its difficulties. Former sea-margins of very recent geological age
occur in all latitudes, and if we are to explain these by sub-oceanic
depression, this will compel us to admit, as Suess has remarked, a
general lowering of the sea-level of upwards of 1000 feet. But it is
difficult to believe that the sea-floor could have subsided to such an
extent in recent times. Suess thinks it is much more probable that the
high-level beaches of tropical regions are not contemporaneous with
those of higher latitudes, and that the phenomena are best explained
by his hypothesis of a secular movement of the ocean--the water being,
as he contends, alternately heaped up at the equator and the poles.
The strand-lines in high latitudes, however, are certainly connected
with glaciation in some way not yet understood; and if it cannot be
confidently affirmed that they indicate regional movements of the land,
the evidence, nevertheless, seems to point in that direction.
In concluding this imperfect outline-sketch of a large subject, I
ought perhaps to apologise for having trespassed so much upon the
domains of geology. But in doing so I have only followed the example
of geologists themselves, whose divagations in territories adjoining
their own are naturally not infrequent. From much that I have said, it
will be gathered that with regard to the causes of many coastal changes
we are still groping in the dark. It seems not unlikely, however, that
as light increases we may be compelled to modify the view that all
oscillations of the sea-level are due to movements of the lithosphere
alone. That is a very heretical suggestion; but that a great deal can
be said for it any one will admit after a candid perusal of Suess's
monumental work, _Das Antlitz der Erde_.
[Illustration:
PLATE VI
BATHY-HYPSOMETRICAL MAP OF THE WORLD
NOTE
_The map is coloured to show the surface relief of the Globe
without water in the Ocean Basins._
EXPLANATION OF COLOURING
_The Colouring is graded from the Darkest Tint at the Highest Level
to the Lightest Tint on the Lowest._
The Edinburgh Geographical Institute J. G. Bartholomew F.R.G.S.
]
EDINBURGH
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32 YORK PLACE.
THE END.
Other Works by Professor James Geikie.
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Transcriber's Note
Hyphenation was not standardized. The section labeled "Explanation of
Plate III." (p. 325) in the original printed version appears to describe
Plate IV and changed accordingly. Paragraphs split by illustrations were
rejoined. Some plates were moved to end of the chapter.