The geology of Asia is so complex and over wide areas so little known
that it is difficult to give a connected account of either the
structure or the development of the continent, and only the broader
features can be dealt with here.
In the south, in Syria, Arabia and the peninsula of India, none but
the oldest rocks are folded, and the Upper Palaeozoic, the Mesozoic
and the Tertiary beds lie almost horizontally upon them. It is a
region of quiescence or of faulting, but not of folding. North of this
lies a broad belt in which the Mesozoic deposits and even the lower
divisions of the Tertiary system are thrown into folds which extend in
a series of arcs from west to east and now form the principal mountain
ranges of central Asia. This belt includes Asia Minor, Persia,
Afghanistan, Baluchistan, the Himalayas, the Tian-shan, and, although
they are very different in direction, the Burmese ranges. The
Kuen-lun, Nan-shan and the mountain ranges of southern China are,
perhaps, of earlier date, but nevertheless they be in the same belt.
It is not true that throughout the whole width of this zone the beds
are folded. There are considerable tracts which are but little
disturbed, but these tracts are enclosed within the arcs formed by the
folds, and the zone taken as a whole is distinctly one of crumpling.
North of the folded belt, and including the greater part of Siberia,
Mongolia and northern China, lies another area which is, in general,
free from any important folding of Mesozoic or Tertiary age. There
are, it is true, mountain ranges which are formed of folded beds; but
in many cases the direction of the chains is different from that of
the folds, so that the ranges must owe their elevation to other
causes; and the folds, moreover, are of ancient date, for the most
part Archaean or Palaeozoic. The configuration of the region is
largely due to faulting, trough-like or tray-like depressions being
formed, and the intervening strips, which have not been depressed,
standing up as mountain ridges. Over a large part of Siberia and in
the north of China, even the Cambrian beds still lie as horizontally
as they were first laid down. In the extreme north, in the Verkhoyansk
range and in the mountains of the Taimyr peninsula, there are
indications of another zone of folding of Mesozoic or later date, but
our information concerning these ranges is very scanty. Besides the
three chief regions into which the mainland is thus seen to be
divided, attention should be drawn to the festoons of islands which
border the eastern side of the continent, and which are undoubtedly
due to causes similar to those which produced the folds of the folded
belt.
Of all the Asiatic ranges the Himalayan is, geologically, the best
known; and the evidence which it affords shows clearly that the folds
to which it owes its elevation were produced by an overthrust from
the north. It is, indeed, as if the high land of central Asia had been
pushed southward against and over the unyielding mass formed by the
old rocks of the Indian peninsula, and in the process the edges of the
over-riding strata had been crumpled and folded. Overlooking all
smaller details, we may consider Asia to consist of a northern mass
and a southern mass, too rigid to crumple, but not too strong to
fracture, and an intermediate belt of softer rock which was capable of
folding. If then by the contraction of the earth's interior the outer
crust were forced to accommodate itself to a smaller nucleus, the
central softer belt would yield by crumpling, the more rigid masses to
the north and south, if they gave way at all, would yield by faulting.
It is interesting to observe, as will be shown later, that during the
Mesozoic era there was a land mass in the north of Asia and another in
the south, and between them lay the sea in which ordinary marine
sediments were deposited. The belt of folding does not precisely
coincide with this central sea, but the correspondence is fairly
close.
The present outline of the eastern coast and the nearly enclosed seas
which lie between the islands and the mainland, are attributed by
Richthofen chiefly to simple faulting.
Little is known of the early geological history of Asia beyond the
fact that a large part of the continent was covered by the sea during
the Cambrian and Ordovician periods. But there is positive evidence
that much of the north and east of Asia has been land since the
Palaeozoic era, and it has been conclusively proved that the peninsula
of India has never been beneath the sea since the Carboniferous period
at least. Between these ancient land masses lies an area in which
marine deposits of Mesozoic age are well developed and which was
evidently beneath the sea during the greater part of the Mesozoic era.
The northern land mass has been named Angaraland by E. Suess; the
southern, of which the Indian peninsula is but a fragment, is called
Gondwanaland by Neumayr, Suess and others, while the intervening sea
is the central Mediterranean sea of Neumayr and the Tethys of Suess.
The greater part of western Asia, including the basin of the Obi, the
drainage area of the Aral Sea, together with Afghanistan, Baluchistan,
Persia and Arabia, was covered by the sea during the later stages of
the Cretaceous period, but a considerable part of this region was
probably dry land in Jurassic times.
The northern land mass begins in the north with the area which lies
between the Yenisei and the Lena. Here the folded Archean rocks are
overlaid by Cambrian and Ordovician beds, which still lie for the most
part flat and undisturbed. Upon these rest patches of freshwater
deposits containing numerous remains of plants. They consist chiefly
of sandstone and conglomerate, but include workable seams of coal.
Some of the deposits appear to be of Permian age, but others are
probably Jurassic, and they are all included under the general name of
the Angara series. Excepting in the extreme north, where marine
Jurassic and Cretaceous fossils have been found, there is no evidence
that this part of Siberia has been beneath the sea since the early
part of the Palaeozoic era. Besides the plant beds extensive outflows
of basic lava rest directly upon the Cambrian and Ordovician strata.
The date of these eruptions is still uncertain, but they probably
continued to a very recent period.
South and east of the Palaeozoic plateau is an extensive area
consisting chiefly of Archean rocks, and including the greater part of
Mongolia north of the Tian-shan. Here again there are no marine beds
of Mesozoic or Tertiary age, while plant-bearing deposits belonging to
the Angara series are known. Structurally, the folds of this region
are of ancient date, but the area is crossed by a series of
depressions formed by faults, and the intervening strips, which have
not been depressed to the same extent, now stand up as mountain
ranges. Farther south, in the Chinese provinces of Shansi and Shensi,
the geological succession is similar in some respects to that of the
Siberian Palaeozoic plateau, but the sequence is more complete. There
is again a floor of folded Archean rocks overlaid by nearly horizontal
strata of Lower Palaeozoic age, but these are followed by marine beds
belonging to the Carboniferous period. From the Upper Carboniferous
onward, however, no marine deposits are known; and, as in Siberia,
plant bearing beds are met with. Southern China is very different in
structure, consisting largely of folded mountain chains; but the
geological succession is very similar, and excepting near the Tibetan
and Burmese borders, there are no marine deposits of Mesozoic or
Tertiary age.
Thus it appears that from the Arctic Ocean there stretches a broad
area as far as the south of China, in which no marine deposits of
later date than Carboniferous have yet been found, except in the
extreme north. Freshwater and terrestrial deposits of Mesozoic age
occur in many places, and the conclusion is irresistible that the
greater part of this area has been land since the close of the
Palaeozoic era. The Triassic deposits of the Verkhoyansk Range show
that this land did not extend to the Bering Sea, while the marine
Mesozoic deposits of Japan on the east, the western Tian-shan on the
west and Tibet on the south give us some idea of its limits in other
directions.
In the same way the entire absence of any marine fossils in the
peninsula of India, excepting near its borders, and the presence of
the terrestrial and freshwater deposits of the Gondwana series,
representing the whole of the geological scale from the top of the
Carboniferous to the top of the Jurassic, show that this region also
has been land since the Carboniferous period. It was a portion of a
great land mass which probably extended across the Indian Ocean and
was at one time united with the south of Africa.
But these two land masses were not connected. Between India and China
there is a broad belt in which marine deposits of Mesozoic and
Tertiary age are well developed. Marine Tertiary beds occur in Burma;
in the Himalayas and in south Tibet there is a nearly complete series
of marine deposits from the Carboniferous to the Eocene; in
Afghanistan the Mesozoic beds are in part marine and in part
fluviatile. The sea in which these strata were deposited seems to have
attained its greatest extension in Upper Cretaceous times when its
waters spread over the whole of western Asia and even encroached
slightly upon the Indian land. The Eocene sea however cannot have been
much inferior in extent.
It was after the Eocene period that the main part of the elevation of
the Himalayas took place, as is shown by the occurrence of nummulitic
limestone at a height of 20,000 ft. The formation of this and of the
other great mountain chains of central Asia resulted in the isolation
of portions of the former central sea, and the same forces finally led
to the elevation of the whole region and the union of the old
continents of Angara and Gondwana. Gondwanaland, however, did not long
survive, and the portion which lay between India and South Africa sank
beneath the waves in Tertiary times.
Leaving out of consideration all evidence of more ancient volcanic
activity, each of the three regions into which, as we have seen, the
continent may be divided has been, during or since the Cretaceous
period, the seat of great volcanic eruptions. In the southern region
of unfolded beds are found the lavas of the "harras" of Arabia, and in
India the extensive flows of the Deccan Trap. In the central folded
belt lie the great volcanoes, now mostly extinct, of Asia Minor,
Armenia, Persia and Baluchistan. In Burma also there is at least one
extinct volcano. In the northern unfolded region great flows of basic
lava lie directly upon the Cambrian and Ordovician beds of Siberia,
but are certainly in part of Tertiary age. Similar flows on a smaller
scale occur in Manchuria, Korea and northern China.
In all these cases, however, the eruptions have now almost ceased, and
the great volcanoes of the present day lie in the islands off the
eastern and south eastern coasts.
REFERENCES--E. Suess, _Das Antlitz der Erde_ (see, especially, vol.
iii. part 1.); F.V. Richthofen, "Ueber Gestalt und Gliederung einer
Grundlinie in der Morphologie Ost-Asiens," _Sitz. k. preuss. Akad.
Wiss._ (Berlin, 1900), pp. 888-925, and Geomorphologische Studien aus
Ostasien, _ibid._, 1901, pp. 782-808, 1902, pp. 944-975, 1903, pp.
867-918. (P. La.)