co-extensive with, the area of dominant elevation, for very
considerable portions of the continental plateau are still covered by
the sea. Opinions may differ as to which fathoms-line we should take as
marking approximately the boundary between that region and the oceanic
depression; and it is obvious, indeed, that any line selected must be
arbitrary and more or less misleading, for it is quite certain that
the true boundary of the continental plateau cannot lie parallel to
the surface of the ocean. In some regions it approaches within a few
hundreds of fathoms of the sea-level; in other places it sinks for
considerably more than 1000 fathoms below that level. Thus, while a
very moderate elevation would in certain latitudes cause the land to
extend to the edge of the plateau, an elevation of at least 10,000 feet
would be required in some other places to bring about a similar result.
Although it is true that the land-surface is nowhere co-extensive with
the great plateau, yet the existing coast-lines may be said to trend
in the same general direction as its margins. So abruptly does the
continental plateau rise from the oceanic trough, that a depression
of the sea-level, or an elevation of the plateau, for 10,000 feet,
would add only a narrow belt to the Pacific coast between Alaska and
Cape Horn, while the gain of land on the Atlantic slope of America
between 30° N.L. and 40° S.L. would not be much greater. In the higher
latitudes of the northern hemisphere, however, very considerable
geographical changes would be accomplished by a much less amount of
elevation of the plateau. Were the continental plateau to be upheaved
for 3000 feet, the major portion of the Arctic Sea would become land.
Thus, in general terms, we may say that the coast-lines of arctic and
temperate North America and Eurasia are further withdrawn from the edge
of the continental plateau than those of lower latitudes.
In regions where existing coast-lines approach the margin of the
plateau, they are apt to run for long distances in one determinate
direction, and, whether the coastal area be high or not, to show a
gentle sinuosity. Their course is seldom interrupted by bold projecting
headlands or peninsulas, or by intruding inlets, while fringing or
marginal islands rarely occur. To these appearances the northern
regions, as every one knows, offer the strongest contrast. Not only do
they trend irregularly, but their continuity is constantly interrupted
by promontories and peninsulas, by inlets and fiords, while fringing
islands abound. But an elevation of some 400 or 500 fathoms only
would revolutionise the geography of those regions, and confer upon
the northern coast-lines of the world the regularity which at present
characterises those of western Africa.
It is obvious, therefore, that the coast-lines of such lands as Africa
owe their regularity primarily to their approximate coincidence with
the steep boundary-slopes of the continental plateau, while the
irregularities characteristic of the coast-line of north-western Europe
and the corresponding latitudes of North America are determined by the
superficial configuration of the same plateau, which in those regions
is relatively more depressed. I have spoken of the general contrast
between high and low northern latitudes; but it is needless to say
that in southern regions the coast-lines exhibit similar contrasts.
The regular coast-lines of Africa and South America have already been
referred to; but we cannot fail to recognise in the much-indented
sea-board and the numerous coastal islands of southern Chile a complete
analogy to the fiord regions of high northern latitudes. Both are areas
of comparatively recent depression. Again, the manifold irregularities
of the coasts of south-eastern Asia, and the multitudes of islands
that serve to link that continent to Australia and New Zealand, are all
evidence that the surface of the continental plateau in those regions
is extensively invaded by the sea.
A word or two now as to the configuration of the oceanic trough. There
can be no doubt that this differs very considerably from that of the
land-surface. It is, upon the whole, flat or gently-undulating. Here
and there it swells gently upwards into broad elevated banks, some of
which have been traced for great distances. In other places narrower
ridges and abrupt mountain-like elevations diversify its surface,
and project again and again above the level of the sea, to form the
numerous islets of Oceania. Once more, the sounding-line has made us
acquainted with the notable fact that numerous deep depressions--some
long and narrow, others relatively short and broad--stud the floor
of the great trough. I shall have occasion to refer again to these
remarkable depressions, and need at present only call attention to
the fact that they are especially well-developed in the region of
the western Pacific, where the floor of the sea, at the base of the
bounding slopes of the continental plateau, sinks in places to depths
of three and even of five miles below the existing coast-lines. One may
further note the fact that the deepest areas of the Atlantic are met
with in like manner close to the walls of the plateau--a long ridge,
which rises midway between the continents and runs in the same general
direction as their coast-lines, serving to divide the trough of the
Atlantic into two parallel hollows.
But, to return to our coast-lines and the question of their
development, it is obvious that their general trends have been
determined by crustal movements. Their regularity is in direct
proportion to the closeness of their approach to the margin of the
continental plateau. The more nearly they coincide with the edge of
that plateau, the fewer irregularities do they present; the further
they recede from it, the more highly are they indented. Various
other factors, it is true, have played a more or less important part
in their development, but their dominant trends were undoubtedly
determined at a very early period in the world's history--their
determination necessarily dates back, in short, to the time when the
great world-ridges and oceanic troughs came into existence. So far as
we can read the story told by the rocks, however, it would seem that
in the earliest ages of which geology can speak with any confidence,
the coast-lines of the world must have been infinitely more irregular
than now. In Palæozoic times, relatively small areas of the continental
plateau appeared above the level of the sea. Insular conditions
everywhere prevailed. But as ages rolled on, wider and wider tracts of
the plateau were exposed, and this notwithstanding many oscillations of
level. So that one may say there has been, upon the whole, a general
advance from insular to continental conditions. In other words, the
sea has continued to retreat from the surface of the continental
plateau. To account for this change, we must suppose that depression
of the crust has been in excess within the oceanic area, and that now
and again positive elevation of the continental plateau has taken
place, more especially along its margins. That movements of elevation,
positive or negative, have again and again affected our land-areas can
be demonstrated, and it seems highly probable, therefore, that similar
movements may have been experienced within the oceanic trough.
Two kinds of crustal movement, as we have seen, are recognised by
geologists. Sometimes the crust appears to rise, or, as the case may
be, to sink over wide regions, without much disturbance or tilting
of strata, although these are now and again more or less extensively
fractured and displaced. It may conduce to clearness if we speak of
these movements as regional. The other kind of crustal disturbance
takes place more markedly in linear directions, and is always
accompanied by abrupt folding and mashing together of strata, along
with more or less fracturing and displacement. The plateau of the
Colorado has often been cited as a good example of regional elevation,
where we have a wide area of approximately horizontal strata apparently
uplifted without much rock-disturbance, while the Alps or any other
chain of highly-flexed and convoluted strata will serve as an example
of what we may term axial or linear uplifts. It must be understood
that both regional and axial movements result from the same cause--the
adjustment of the solid crust to the contracting nucleus--and that the
term _elevation_, therefore, is only relative. Sometimes the sinking
crust gets relief from the enormous lateral pressure to which it is
subjected by crumpling up along lines of weakness, and then mountains
of elevation are formed; at other times, the pressure is relieved by
the formation of broader swellings, when wide areas become uplifted
relatively to surrounding regions. Geologists, however, are beginning
to doubt whether upheaval of the latter kind can affect a broad
continental area. Probably, in most cases, the apparent elevation of
continental regions is only negative. The land appears to have risen
because the floor of the oceanic basin has become depressed. Even the
smaller plateau-like elevations which occur within some continental
regions may in a similar way owe their dominance to the sinking of
contiguous regions.
In the geographical development of our land, movements of elevation
and depression have played an important part. But we cannot ignore the
work done by other agents of change. If the orographical features of
the land everywhere attest the potency of plutonic agents, they no less
forcibly assure us that the inequalities of surface resulting from such
movements are universally modified by denudation and sedimentation.
Elevated plains and mountains are gradually demolished, and the hollows
and depressions of the great continental plateau become slowly filled
with their detritus. Thus inland-seas tend to vanish, inlets and
estuaries are silted up, and the land in places advances seaward. The
energies of the sea, again, come in to aid those of rain and rivers,
so that under the combined action of all the superficial agents of
change, the irregularities of coast-lines become reduced, and, were no
crustal movement to intervene, would eventually disappear. The work
accomplished by those agents upon a coast-line is most conspicuous in
regions where the surface of the continental plateau is occupied by
comparatively shallow seas. Here full play is given to sedimentation
and marine erosion, while the latter alone comes into prominence upon
shores that are washed by deeper waters. When the coast-lines advance
to the edge of the continental plateau, they naturally trend, as we
have seen, for great distances in some particular direction. Should
they preserve that position, undisturbed by crustal oscillation, for a
prolonged period of time, they will eventually be cut back by the sea.
In this way a shelf or terrace will be formed, narrow in some places,
broader in others, according to the resistance offered by the varying
character of the rocks. But no long inlets or fiords can result from
such action. At most the harder and less readily demolished rocks will
form headlands, while shallow bays will be scooped out of the more
yielding masses. In short, between the narrower and broader parts
of the eroded shelf or terrace a certain proportion will tend to be
preserved. As the shelf is widened, sedimentation will become more and
more effective, and in places may come to protect the land from further
marine erosion. This action is especially conspicuous in tropical and
sub-tropical regions, which are characterised by well-marked rainy
seasons. In such regions immense quantities of sediment are washed down
from the land to the sea, and tend to accumulate along shore, forming
low alluvial flats. All long-established coast-lines thus acquire a
characteristically sinuous form, and perhaps no better examples could
be cited than those of western Africa.
To sum up, then, we may say that the chief agents concerned in the
development of coast-lines are crustal movements, sedimentation, and
marine erosion. All the main trends are the result of elevation and
depression. Considerable geographical changes, however, have been
brought about by the silting up of those shallow and sheltered seas
which, in certain regions, overflow wide areas of the continental
plateau. Throughout all the ages, indeed, epigene agents have striven
to reduce the superficial inequalities of that plateau, by levelling
heights and filling up depressions, and thus, as it were, flattening
out the land-surface and causing it to extend. The erosive action
of the sea, from our present point of view, is of comparatively
little importance. It merely adds a few finishing touches to the work
performed by the other agents of change.
A glance at the geographical evolution of our own Continent will render
this sufficiently evident. Viewed in detail, the structure of Europe
is exceedingly complicated, but there are certain leading features in
its architecture which no profound analysis is required to detect. We
note, in the first place, that highly-disturbed rocks of Archæan and
Palæozoic age reach their greatest development along the north-western
and western borders of our Continent, as in Scandinavia, the British
Islands, north-west France, and the Iberian peninsula. Another belt
of similarly disturbed strata of like age traverses central Europe
from west to east, and is seen in the south of Ireland, Cornwall,
north-west France, the Ardennes, the Thüringer-Wald, the Erz Gebirge,
the Riesen Gebirge, the Böhmer-Wald, and other heights of middle and
southern Germany. Strata of Mesozoic and Cainozoic age rest upon the
older systems in such a way as to show that the latter had been much
folded, fractured, and denuded before they came to be covered with
younger formations. North and north-east of the central belt of ancient
rocks just referred to, the sedimentary strata that extend to the
shores of the Baltic and over a vast region in Russia, range in age
from Palæozoic down to Cainozoic times, and are disposed for the most
part in gentle undulations--they are either approximately horizontal
or slightly inclined. Unlike the disturbed rocks of the maritime
regions and of central Europe, they have obviously been subjected to
comparatively little folding since the time of their deposition. To
the south of the primitive back-bone of central Europe succeeds a
region composed superficially of Mesozoic and Cainozoic strata for the
most part, which, along with underlying Palæozoic and Archæan rocks,
are often highly-flexed and ridged up, as in the chains of the Jura,
the Alps, the Carpathians, etc. One may say, in general terms, that
throughout the whole Mediterranean area Archæan and Palæozoic rocks
appear at the surface only when they form the nuclei of mountains of
elevation, into the composition of which rocks of younger age largely
enter.
From this bald and meagre outline of the general geological structure
of Europe, we may gather that the leading orographical features of our
Continent began to be developed at a very early period. Unquestionably
the oldest land-areas are represented by the disturbed Archæan
and Palæozoic rocks of the Atlantic sea-board and central Europe.
Examination of those tracts shows that they have experienced excessive
denudation. The Archæan and Palæozoic masses, distributed along the
margin of the Atlantic, are the mere wrecks of what, in earlier ages,
must have been lofty regions, the mountain-chains of which may well
have rivalled or even exceeded in height the Alps of to-day. They,
together with the old disturbed rocks of central Europe, formed for a
long time the only land in our area. Between the ancient Scandinavian
tract in the north and a narrow interrupted belt in central Europe,
stretched a shallow sea, which covered all the regions that now form
our Great Plain; while immediately south of the central belt lay the
wide depression of the Mediterranean--for as yet the Pyrenees, the
Alps, and the Carpathians were not. Both the Mediterranean and the
Russo-Germanic sea communicated with the Atlantic. As time went on land
continued to be developed along the same lines, a result due partly
to crustal movements, partly to sedimentation. Thus the relatively
shallow Russo-Germanic sea became silted up, while the Mediterranean
shore-line advanced southwards. It is interesting to note that the
latter sea, down to the close of Tertiary times, seems always to have
communicated freely with the Atlantic, and to have been relatively
deep. The Russo-Germanic sea, on the contrary, while now and again
opening widely into the Atlantic, and attaining considerable depths in
its western reaches, remained on the whole shallow, and ever and anon
vanished from wide areas to contract into a series of inland-seas and
large salt lakes.
Reduced to its simplest elements, therefore, the structure of Europe
shows two primitive ridges--one extending with some interruptions
along the Atlantic sea-board, the other traversing central Europe
from west to east, and separating the area of the Great Plain from
the Mediterranean basin. The excessive denudation which the more
ancient lands have undergone, and the great uplifts of Mesozoic and of
Cainozoic times, together with the comparatively recent submergence
of broad tracts in the north and north-west, have not succeeded in
obscuring the dominant features in the architecture of our Continent.
I now proceed to trace, as rapidly as I can, the geographical
development of the coast-lines of the Atlantic as a whole, and to
point out the chief contrasts between them and the coast-lines of the
Pacific. The extreme irregularity of the Arctic and Atlantic shores of
Europe at once suggests to a geologist a partially-drowned land, the
superficial inequalities of which are accountable for the vagaries of
the coast-lines. The fiords of Norway and Scotland occupy what were
at no distant date land-valleys, and the numerous marginal islands of
those regions are merely the projecting portions of a recently-sunken
area. The continental plateau extends up to and a little beyond the
one hundred fathoms line, and there are many indications that the
land formerly reached as far. Thus the sunken area is traversed by
valley-like depressions, which widen as they pass outwards to the
edge of the plateau, and have all the appearance of being hollows
of sub-aërial erosion. I have already mentioned the fact that the
Scandinavian uplands and the Scottish Highlands are the relics of what
were at one time true mountains of elevation, corresponding in the mode
of their formation to those of Switzerland, and, like these, attaining
a great elevation. During subsequent stages of Palæozoic time, that
highly-elevated region was subjected to long-continued and profound
erosion--the mountain-country was planed down over wide regions to
sea-level, and broad stretches of the reduced land-surface became
submerged. Younger Palæozoic formations then accumulated upon the
drowned land, until eventually renewed crustal disturbance supervened,
and the marginal areas of the continental plateau again appeared as
dry land, but not, as before, in the form of mountains of elevation.
Lofty table-lands now took the place of abrupt and serrated ranges
and chains--table-lands which, in their turn, were destined in the
course of long ages to be deeply sculptured and furrowed by sub-aërial
agents. During this process the European coast-line would seem to
have coincided more or less closely with the edge of the continental
plateau. Finally, after many subsequent movements of the crust in
these latitudes, the land became partially submerged--a condition from
which north-western and northern Europe would appear in recent times
to be slowly recovering. Thus the highly-indented coast-line of those
regions does not coincide with the edge of the plateau, but with those
irregularities of its upper surface which are the result of antecedent
sub-aërial erosion.
Mention has been made of the Russo-Germanic plain and the Mediterranean
as representing original depressions in the continental plateau, and
of the high-grounds that extend between them as regions of dominant
elevation, which, throughout all the manifold revolutions of the past,
would appear to have persisted as a more or less well-marked boundary,
separating the northern from the southern basin. During certain periods
it was no doubt in some degree submerged, but never apparently to
the same extent as the depressed areas it served to separate. From
time to time uplifts continued to take place along this central belt,
which thus increased in breadth, the younger formations, which were
accumulated along the margins of the two basins, being successively
ridged up against nuclei of older rocks. The latest great crustal
movements in our Continent, resulting in the uplift of the Alps and
other east and west ranges of similar age, have still further widened
that ancient belt of dominant elevation which in our day forms the most
marked orographical feature of Europe.
The Russo-Germanic basin is now for the most part land, the Baltic
and the North Sea representing its still submerged portions. This
basin, as already remarked, was probably never so deep as that of the
Mediterranean. We gather as much from the fact that, while mechanical
sediments of comparatively shallow-water origin predominate in the
former area, limestones are the characteristic features of the
southern region. Its relative shallowness helps us to understand why
the northern depression should have been silted up more completely
than the Mediterranean. We must remember also that for long ages it
received the drainage of a much more extensive land-surface than the
latter--the land that sloped towards the Mediterranean in Palæozoic
and Mesozoic times being of relatively little importance. Thus the
crustal movements which ever and anon depressed the Russo-Germanic area
were, in the long-run, counterbalanced by sedimentation. The uplift
of the Alps, the Atlas, and other east and west ranges, has greatly
contracted the area of the Mediterranean, and sedimentation has also
acted in the same direction, but it is highly probable that that sea
is now as deep as, or even deeper than, it has ever been. It occupies
a primitive depression in which the rate of subsidence has exceeded
that of sedimentation. In many respects, indeed, this remarkable
transmeridional hollow--continued eastward in the Red Sea, the Black
Sea, and the Aralo-Caspian depression--is analogous, as we shall see,
to the great oceanic trough itself.
In the earlier geological periods linear or axial uplifts and
volcanic action again and again marked the growth of land on the
Atlantic sea-board. But after Palæozoic times, no great mountains of
elevation came into existence in that region, while volcanic action
almost ceased. In Tertiary times, it is true, there was a remarkable
recrudescence of volcanic activity, but the massive eruptions of
Antrim and western Scotland, of the Faröe Islands and Iceland, must
be considered apart from the general geology of our Continent. From
Mesozoic times onwards it was along the borders of the Mediterranean
depression that great mountain uplifts and volcanoes chiefly presented
themselves; and as the land-surface extended southwards from central
Europe, and the area of the Mediterranean was contracted, volcanic
action followed the advancing shore-lines. The occurrence of numerous
extinct and of still existing volcanoes along the borders of this
inland-sea, the evidence of recent crustal movements so commonly
met with upon its margins, the great irregularities of its depths,
the proximity of vast axial uplifts of late geological age, and the
frequency of earthquake phenomena, all indicate instability, and remind
us strongly of similarly constructed and disturbed regions within the
area of the vast Pacific.