the first class. These consist chiefly of the more or less indurated
sediments of ancient rivers, lakes, and seas--namely, conglomerate,
sandstone, shale, limestone, etc. And now and again, interstratified
with such aqueous beds, we meet with rocks of terrestrial origin, such
as lignite, coal, and the débris of former glacial action. Now, most of
our aqueous rocks have been accumulated in the sea, and thus we arrive
at the conclusion that the present continental areas have from time to
time been largely submerged--that the sea has frequently covered what
are now the dry lands of the globe. But one remarkable fact stands out,
and it is this: Nowhere amongst the sedimentary rocks of the earth's
crust do we meet with any ancient sediments which can be likened to
the red clay now slowly accumulating in the deeper abysses of the
ocean. There are no rocks of abysmal origin. Many of our limestones
have undoubtedly formed in deep, clear water, but none of these is
abysmal. Portions of Europe may now and again have been submerged for
several thousand feet, but no part of this or any other continent,
so far as we yet know, has within geological time been depressed to
depths comparable to those of the present oceanic basins. Nay, by
far the larger portions of our marine formations have accumulated in
comparatively shallow water--sandstones and shales (sand and mud) being
by far the most common kinds of rock that we encounter. In short,
aqueous strata have, as a rule, been deposited at no great depth and
at no great distance from dry land; the rocks are built up mostly of
terrigenous material; and even the purer limestones and chalks, which
we may suppose accumulated in seas of moderate depth, not infrequently
contain some terrestrial relic which has been drifted out to sea, and
afford other evidence to show that the nearest land was never very far
away. Followed along their outcrop such rocks sooner or later become
mixed and interbedded with ordinary sedimentary matter. Thus, for
example, the thick carboniferous limestone of Wales and the Midlands of
England must have accumulated in the clear water of a moderately deep
sea. But when this limestone is traced north into Northumberland it
begins to receive intercalations of sandstone and shale, which become
more and more important, until in Scotland they form by much the larger
portion of the series--the enormous thick limestones of the south being
represented by only a few inconsiderable beds, included, along with
seams of ironstone and coal, in a thick succession of sandstones and
shales.
Of the igneous rocks and the crystalline schists I need not speak at
present, but I shall have something to say about them before I have
done.
Having learned that no truly abysmal rocks enter into the composition
of our continents, of what kind of rocks, we may ask, are the islands
composed? Well, some of those islands are built up of precisely the
same materials as we find in the continents. This is the case with most
islands which are not separated from continental areas by profoundly
deep seas. Thus our own islands with their numerous satellites are
geologically one with the adjacent continent. Their present separation
is a mere accident. Were the European area, with the adjacent sea-bed,
to be elevated for a few hundred feet we should find that Britain and
Ireland form geologically part and parcel of the continent. And the
same is the case with Nova Zembla and Spitzbergen in the north, and
with the Mediterranean islands in the south. There is another large
class of islands, however, which are characterised by the total absence
of any of those sedimentary rocks of which, as I have just said, our
continents and continental islands are chiefly built up. The islands
referred to are scattered over the bosom of the great ocean, and are
surrounded by profoundly deep water. Some are apparently composed
entirely of coral, others are of volcanic origin, and yet others are
formed partly of volcanic rock and partly of coral. Thus we have two
distinct kinds of island:--
1st, Islands which have at one time evidently been connected with
adjacent continents, and which are therefore termed
_continental islands_; and
2d, _Oceanic islands_, which rise, as it were, from profound depths
in the sea, and which have never formed part of the continents.
The fauna and flora of the former class agree with those of the
neighbouring continents, although some modifications are met with,
especially when the insulation has been of long standing. When such has
been the case the species of plants and animals may be almost entirely
distinct. Nevertheless, such ancient continental islands agree with
those which have been separated in more recent geological times in
containing both indigenous amphibians and mammals. Oceanic islands,
on the other hand, contain no indigenous mammals or amphibians,
their life consisting chiefly of insects and birds, and usually some
reptiles--just such a fauna as might have been introduced by the
influence of winds and of oceanic currents carrying driftwood.
Such facts, as have now been briefly summarised, point clearly to
the conclusion that the oceanic basins and continental areas are of
primeval antiquity. All the geological facts go to prove that abysmal
waters have never prevailed over the regions now occupied by dry land;
nor is there any evidence to show that continental land-masses ever
existed in what are now the deepest portions of the ocean. The islets
dotted over the surface of the Pacific and the other great seas are
not the relics of a vast submerged continent. They are either the tops
of submarine volcanic mountains, or they are coral structures founded
upon the shoulders of degraded volcanoes and mountain-chains, and
built up to the surface by the indefatigable labours of the humble
polyp. We come then to the general conclusion that oceanic basins and
intervening continental ridges are great primeval wrinkles in the
earth's crust--that they are due to the sinking down of that crust
upon the cooling and contracting nucleus. These vast wrinkles had come
into existence long before the formation of our oldest geological
strata. All our rocks may, in short, be looked upon as forming a mere
superficial skin covering and concealing the crystalline materials
which no doubt formed the original surface of the earth's crust.
Having premised so much, let me now turn to consider the geological
history of our own Continent, and endeavour to trace out the various
stages in its evolution. Of course I can only do so in a very brief
and general manner; it is impossible to go into details. We shall
find, however, that the history of the evolution of Europe, even
when sketched in outline, is one full of instruction for students
of physical geography, and that it amply bears out the view of the
permanency of the greater features of the earth's surface.
The oldest rocks that we know of are the crystalline schists and
gneiss, belonging to what is called the Archæan system. The origin
of these rocks is still a matter of controversy--some holding them
to be part of the primeval crust of the globe, or the chemical
precipitates of a primeval ocean, others maintaining that they are
altered or metamorphosed rocks of diverse origin, a large proportion
having consisted originally of aqueous or sedimentary rocks, such
as sandstone and shale; while not a few are supposed to have been
originally eruptive igneous rocks. According to some geologists,
therefore, the Archæan rocks represent the earliest sediments
deposited over the continental ridges. It is supposed that here and
there those ridges rose above the surface of what may have been a
boiling or highly-heated ocean, from whose waters copious chemical
precipitations took place, while gravel and shingle gathered around
the shores of the primeval lands. According to other writers, however,
the Archæan rocks were probably accumulated under normal conditions.
They consist, it is contended, partly of sediment washed down from
some ancient land-surface, and distributed over the floor of an old
sea (just as sediments are being transported and deposited in our own
day), and partly of ancient igneous rocks. Their present character is
attributed to subsequent changes, superinduced by heat and pressure,
at a time when the masses in question were deeply buried under later
formations, which have since been washed away. In a word, we are
still quite uncertain as to the true origin of the Archæan rocks. Not
infrequently they show a bedded structure, and in that respect they
simulate the appearance of strata of sedimentary origin. It is very
doubtful, however, whether this "bedded structure" is any evidence of
an original aqueous arrangement. We know now that an appearance of
bedding has been induced in originally amorphous rocks during great
earth-movements. Granite masses, for example, have been so crushed
and squeezed as to assume a bedded aspect, and a similar structure
has been developed in many other kinds of rock subjected to enormous
pressure. Whatever may have been the origin of the bedded structure
of the Archæan rocks, it is certain that the masses have been tilted
up and convoluted in the most remarkable manner. Hitherto they have
yielded no unequivocal trace of organic remains--the famous _Eozoon_
being now generally considered as of purely mineral origin. The
physical conditions under which the Archæan gneiss and schist came into
existence are thus quite undetermined, but geologists are agreed that
the earliest land-surfaces, of the former existence of which we can
be quite certain, were composed of rocks. And this brings us to the
beginning of reliable geological history.
All subsequent geological time--that, namely, of which we have any
record preserved in the fossiliferous strata--is divided into four
great eras, namely the Palæozoic, the Mesozoic, the Cainozoic, and
the Post-Tertiary eras, each of which embraces various periods, as
follows:--
Post-Tertiary {Recent.
{Pleistocene.
{Pliocene.
Tertiary or {Miocene.
Cainozoic {Oligocene.
{Eocene.
{Cretaceous.
Secondary or {Jurassic.
Mesozoic {Triassic.
{Permian.
{Carboniferous.
Primary or {Devonian and Old Red Sandstone.
Palæozoic {Silurian.
{Cambrian.
Archæan, Fundamental Gneiss.
Leaving the Archæan, we find that the next oldest strata are those
which were accumulated during the Cambrian period, to which succeeded
the Silurian, the Devonian and Old Red Sandstone, the Carboniferous,
and the Permian periods--all represented by great thicknesses of
strata, which overspread wide regions.
Now, at the beginning of the Cambrian period, we have evidence to show
that the primeval continental ridge was still largely under water,
the dry land being massed chiefly in the north. At that distant date
a broad land-surface extended from the Outer Hebrides north-eastwards
through Scandinavia, Finland, and northern Russia. How much further
north and north-west of the present limits of Europe that ancient
land may have extended we cannot tell, but it probably occupied wide
regions which are now submerged in the shallow waters of the Arctic
Ocean. In the north of Scotland a large inland sea or lake existed in
Cambrian times,[DG] and there is some evidence to suggest that similar
lacustrine conditions may have obtained in the Welsh area at the
beginning of the period. South of the northern land lay a shallow sea
covering all middle and southern Europe. That sea, however, was dotted
here and there with a few islands of Archæan rocks, occupying the site
of what are now some of the hills of middle Germany, such as the Riesen
Gebirge, the Erz Gebirge, the Fichtel Gebirge, etc., and possibly some
of the Archæan districts of France and the Iberian peninsula.
[DG] The Red Sandstones of the north-west Highlands are now believed to
be of pre-Cambrian age.
The succeeding period was one of eminently marine conditions, the wide
distribution of Silurian strata showing that during the accumulation
of these, enormous tracts of our Continent were overflowed by the sea.
None of these deposits, however, is of truly oceanic origin. They
appear for the most part to have been laid down in shallow seas, which
here and there may have been moderately deep. During the formation of
the Lower Silurian the whole of the British area, with the exception
perhaps of some of the Archæan tracts of the north-west, seems to have
been under water. The submergence had commenced in Cambrian times, and
was continued up to the close of the Lower Silurian period. During
this long-continued period of submergence volcanic activity manifested
itself at various points--our country being represented at that time
by groups of volcanic islands, scattered over the site of what is now
Wales, and extending westward into the Irish region, and northwards
into the districts of Cumberland and south Ayrshire. Towards the close
of the Lower Silurian period considerable earth-movements took place,
which had the effect of increasing the amount of dry land, the most
continuous mass or masses of which still occupied the northern and
north-western part of our Continent. In the beginning of Upper Silurian
times a broad sea covered the major portion of middle and probably all
southern Europe. Numerous islands, however, would seem to have existed
in such regions as Wales, and the various tracts of older Palæozoic
and Archæan rocks of middle Germany. Many of these islands, however,
were partially and some entirely submerged before the close of Silurian
times.
The next great period--that, namely, which witnessed the accumulation
of the Devonian and Old Red Sandstone strata--was in some respects
strongly contrasted to the preceding period. The Silurian rocks, as I
have said, are eminently marine. The Old Red Sandstones, on the other
hand, appear to have been accumulated chiefly in great lakes or inland
seas, and they betoken therefore the former existence of extensive
lands, while the contemporaneous Devonian strata are of marine origin.
Towards the close of the Upper Silurian period, then, we know that
considerable upheavals ensued in western and north-western Europe,
and wide stretches of the Silurian sea-bottom were converted into dry
land. The geographical distribution of the Devonian in Europe, and
the relation of that system to the Silurian, show that the Devonian
sea did not cover so broad an expanse as that of the Upper Silurian.
The sea had shallowed, and the area of dry land had increased when
the Devonian strata began to accumulate. In trying to realise the
conditions that obtained during the formation of the Devonian and the
Old Red Sandstone, we may picture to ourselves a time when the Atlantic
Ocean extended eastwards over the south of England and the north-east
of France, and occupied the major portion of central Europe, sweeping
north-east into Russia, and how much further we cannot tell. North
of that sea stretched a wide land-surface, in the hollows of which
lay great lakes or inland seas, which seem now and again to have had
communication with the open ocean. It was in these lakes that the Old
Red Sandstone was accumulated, while the Devonian or marine rocks were
formed in the wide waters lying to the south. Submarine volcanoes were
active at that time in Germany; and similarly in Scotland numerous
volcanoes existed, such as those of the Sidlaw Hills and the Cheviots.
The Carboniferous system contains the record of a long and complex
series of geographical changes, but the chief points of importance
in the present rapid review may be very briefly summed up. In the
earlier part of the period marine conditions prevailed. Thus we find
evidence to show that the sea extended further north than it did
during the preceding Devonian period. During the formation of the
mountain-limestone, a deep sea covered the major portion of Ireland and
England, but shallowed off as it entered the Scottish area. A few rocky
islets were all that represented Ireland and England at that time.
Passing eastwards, the Carboniferous sea appears to have covered the
low-grounds of middle Europe and enormous tracts in Russia. The deepest
part of the sea lay over the Anglo-Hibernian and Franco-Belgian areas;
towards the east it became shallower. Probably the same sea swept over
all southern Europe, but many islands may have diversified its surface,
as in Brittany and central France, in Spain and Portugal, and in the
various areas of older Palæozoic and Archæan rocks in central and
south-west Europe. In the latter stages of the Carboniferous period,
the limits of the sea were much circumscribed, and wide continental
conditions supervened. Enormous marshes, jungles, and forests now
overspread the newly-formed lands. Another feature of the Carboniferous
was the great number of volcanoes--submarine and sub-aërial--which were
particularly abundant in Scotland, especially during the earlier stages
of the period.
The rocks of the Permian period seem to have been deposited chiefly in
closed basins. When, owing to the movement of elevation or upheaval
which took place in late Carboniferous times, the carboniferous
limestone sea had been drained away from extensive areas in central
Europe, wide stretches of sea still covered certain considerable
tracts. These, however, as time went on, were cut off from the main
ocean and converted into great salt lakes. Such inland seas overspread
much of the low-lying tracts of Britain and middle Germany, and they
also extended over a broad space in the north-east of Russia. It was in
these seas that the Permian strata were accumulated. The period, it may
be added, was marked by the appearance of volcanic action in Scotland
and Germany.
So far, then, as our present knowledge goes, that part of the European
continent which was the earliest to be evolved lay towards the
north-west and north. All through the Palæozoic era a land-surface
would seem to have endured in that direction--a land-surface from the
denudation or wearing down of which the marine sedimentary formations
of the bordering regions were derived. But when we reflect on the great
thickness and horizontal extent of those sediments, we can hardly
doubt that the primeval land must have had a much wider range towards
the north and north-west than is the case with modern Europe. The
lands, from which the older Palæozoic marine sediments of the British
Islands and Scandinavia were obtained, must, for the most part, be
now submerged. In later Palæozoic times land began to extend in the
Spanish peninsula, northern France, and middle Europe, the denudation
of which doubtless furnished materials for the elaboration of the
contemporaneous strata of those regions. Southern Europe is so largely
composed of Mesozoic and Cainozoic rocks that we can say very little as
to the condition of that area in Palæozoic times, but the probabilities
are that it continued for the most part under marine conditions.
In few words, then, we may conclude that while after Archæan times
dry land prevailed in the north and north-west, marine conditions
predominated further south. Ever and anon, however, the sea vanished
from wide regions in central Europe, and was replaced by terrestrial
and lacustrine conditions. Further, as none of the Palæozoic marine
strata indicates a deep ocean, but all consist for the most part of
accumulations formed at moderate depths, it follows that there must
have been a general subsidence of our area to allow of their successive
deposition--a subsidence, however, which was frequently interrupted by
long pauses, and sometimes by movements in the opposite direction.
The first period of the Mesozoic era, namely, the Triassic, was
characterised by much the same kind of conditions as obtained towards
the close of Palæozoic times. A large inland sea then covered a
considerable portion of England, and seems to have extended north into
the south of Scotland, and across the area of the Irish Sea into the
north-east of Ireland. Another inland sea extended westward from the
Thüringer-Wald across the Vosges into France, and stretched northwards
from the confines of Switzerland over what are now the low-grounds of
Holland and northern Germany. In this ancient sea the Harz Mountains
formed a rocky island. While terrestrial and lacustrine conditions
thus obtained in central and northern Europe, an open sea existed in
the more southerly regions of the continent. Towards the close of the
period submergence ensued in the English and German areas, and the salt
lakes became connected with the open sea.
During the Jurassic period the regions now occupied in Britain and
Ireland by the older rocks appear to have been chiefly dry land.
Scotland and Ireland, for the most part, stood above the sea-level,
while nearly all England was under water--the hills of Cumberland
and Westmoreland, the Pennine chain, Wales, the heights of Devon and
Cornwall, and a ridge of Palæozoic rocks which underlies London, being
the chief lands in south Britain. The same sea overflowed an extensive
portion of what is now the Continent. The older rocks in the north-west
and north-east of France, and the central plateau of the same country,
formed dry land; all the rest of that country was submerged. In like
manner the sea covered much of eastern Spain. In middle Europe it
overflowed nearly all the low-grounds of north Germany, and extended
far east into the heart of Russia. It occupied the site of the Jura
Mountains, and passed eastward into Bohemia, while on the south side of
the Alps it spread over a large part of Italy, extending eastward so as
to submerge a broad area in Austria-Hungary and the Turkish provinces.
Thus the northern latitudes of Europe continued to be the site of the
chief land-masses, what are now the central and southern portions of
the Continent being a great archipelago with numerous islands, large
and small.
The Jurassic rocks, attaining as they do a thickness of several
thousand feet, point to very considerable subsidence. The movement,
however, was not continuous, but ever and anon was interrupted by
pauses. Taken as a whole, the strata appear to have accumulated in a
comparatively shallow sea, which, however, was sufficiently deep in
places to allow of the growth, in clear water, of coral-reefs.
Towards the close of the Jurassic period a movement of elevation
ensued, which caused the sea to retreat from wide areas, and thus when
the Cretaceous period began the British region was chiefly dry land.
Middle Europe would seem also to have participated in this upward
movement. Eventually, however, subsidence again ensued. Most of what
are now the low-grounds of Britain were submerged, the sea stretching
eastwards over a vast region in middle Europe, as far as the slopes
of the Urals. The deepest part of this sea, however, was in the west,
and lay over England and northern France. Further east, in what are
now Saxony and Bohemia, the waters were shallow, and gradually became
silted up. In the Mediterranean basin a wide open sea existed, covering
large sections of eastern Spain and southern France, overflowing the
site of the Jura Mountains, drowning most of the Alpine Lands, the
Italian peninsula, the eastern borders of the Adriatic, and Greece.
In short, there are good grounds for believing that the Cretaceous
Mediterranean was not only much broader than the present sea, but
that it extended into Asia, overwhelming vast regions there, and
communicated with the Indian Ocean.