THE CHANGES WHICH OCCURRED DURING THE THIRD CONTINENTAL PERIOD IN
BRITAIN; AND THE FOREIGN PERMO-CARBONIFEROUS ROCKS.
At the close of Carboniferous times a marked change took place in the
nature of the earth-movements. The prevalent depression which occurred
over the British and adjoining regions during Carboniferous times was
replaced by upward movement, accompanied by orogenic folds, which once
more brought on continental conditions and developed a series of
mountain ranges. The change is marked even at the close of
Carboniferous times by the abnormal red sandstones of the uppermost
part of the Carboniferous system which are found around Whitehaven in
Cumberland and Rotherham in Yorkshire, as the Whitehaven Sandstone and
Rotherham Red Rock. These movements continued through Permian and
Triassic times, and it is to them and to the climatic conditions of
the periods, that the anomalous nature of the Permo-Triassic deposits
is largely due, as will be shewn in the succeeding chapters. At
present it is our purpose to call attention to the effect of these
movements upon the sediments which had been deposited previously to
their occurrence.
Over the British area, two different systems of orogenic movement can
be detected, producing folds of which the axes run approximately at
right angles to one another. One of these, of which the Pennine system
is the best representative in Britain, caused the production of
elevations having axes in a general north and south direction, and we
may therefore speak of it as the Pennine system of movement, while the
other, which gave rise to folds running in an east and west direction,
is well represented in the Mendip Hills, and may be therefore termed
the Mendip system, though it is more widely known as the Hercynian
system, as, on the Continent, the rocks which are greatly affected by
it form the foundations of the region occupied by the ancient
Hercynian forest.
The effects of these systems were in the main similar; they resulted
in the uplift of parallel belts of country to form hill-ranges with
intervening lowlands, but when studied in detail the movements are
seen to be of a different character. The Pennine system of movements
was of a type which is familiar to the geologists as developed in the
Great Basin Region of the western territories of North America, and
produced what is spoken of as Basin-Range structure. The movements
were of the nature of direct uplift, causing fracture, only
accompanied by folding in a minor degree, and accordingly the hills
are composed of terraced scarps, with one gently sloping side, and one
steep scarp-side, the latter on the upthrow side of the fault, as seen
in fig. 21.
In the Mendip system, the folds were of the Alpine type, which is a
familiar product of lateral pressure, consisting essentially of
overfolds, though these are often complicated by reversed faults.
Of the Pennine system, the Pennine Chain itself furnishes the most
noteworthy example in Britain, but we have indications of other folds
of this system, such as that which runs from the Lake District to the
Ayrshire coast, which is partly concealed as the result of other
movements, and a still more marked one, in the rocks of the Malvern
Hills.
[Illustration: Fig. 21.
_a a´_. One stratum displaced by faults _f f_. _h._ Hills.]
The Mendip system is well shewn in the Mendip Hills, but the remains
of a still more important anticline are seen in South Devon and
Cornwall, separated from the Mendip Hills by the great syncline of
Devon. Another parallel anticline runs from Lancashire to Yorkshire at
right angles to the Pennine Chain and separates the coal-field of
Cumberland and that of Northumberland and Durham, from those of South
Lancashire, and Yorkshire, Notts, and Derbyshire.
On the European continent the Ural Chain is the most important uplift
of the system of which the Pennine Chain forms a minor representative,
while the Hercynian system has caused the compression and stiffening
of many of the Carboniferous and earlier rocks which now rise to the
surface in many parts of central Europe.
The extensive continental area which was the result of these uplifts
not only determined the formation of abnormal deposits, but allowed
the occurrence of a long period of time subsequently to the close of
the Carboniferous period, of which few deposits now exposed in Europe
are representative, and we must accordingly seek other regions in
order to find typical representatives of this _Permo-Carboniferous_
period, of which the strata developed in the Salt Range of India have
been most carefully worked, especially by Dr Waagen, though marine
sediments of the period are known elsewhere, as in Spitsbergen, the
Ural Mountains, China and Australasia; and a group of somewhat
anomalous sediments of this age in parts of India, Australia and South
America is of peculiar interest, on account of the insight as to the
climatic conditions of the times which it affords.
_The Permo-Carboniferous Rocks._ In the Salt Range of the North-West
of India an interesting series of sandstones alternating with
limestones rests unconformably upon lower rocks. The sandstones are
known as the Speckled Sandstones, while the limestones are termed the
_Productus_ Limestones. The Lower and Middle Speckled Sandstones are
succeeded by the Lower _Productus_ Limestone which is separated from
the Lower division of the Middle _Productus_ Limestone by the Upper
Speckled Sandstone; these are all of the Permo-Carboniferous period,
while the upper part of the Middle _Productus_ Limestone and the Upper
_Productus_ Limestone belongs to the Permian period. The fossils,
largely invertebrates, are intermediate in character between those of
Carboniferous and Permian ages. Similar fossils are found in the
marine Permo-Carboniferous beds of the other areas which have been
named above. The Lower Speckled Sandstone is of interest on account of
the occurrence of boulder-beds within it, and this division of the
sandstone has been correlated with the lowest (Talchir) stage of the
Permo-Carboniferous strata of other parts of India, while the other
Speckled Sandstones and those divisions of _Productus_ Limestone which
are referred to the Permo-Carboniferous are correlated with the higher
divisions of other parts.
Special mention is made of the Talchir division, on account of the
occurrence therein of boulder beds which have long been known, and
whose glacial origin was inferred by Dr W. T. Blanford forty years
ago. The accumulations shew signs of having been deposited in water,
but the existence of large subangular, sometimes striated boulders
therein, which must have come from distant sources, and the occasional
occurrence of striated rock surfaces on the strata upon which the
Talchir beds repose unconformably points to ice-action; this would not
be so very remarkable if it were an isolated case, though sufficiently
so, from the comparative nearness of the region to the equator; but
researches conducted in different parts of the southern hemisphere
have brought to light similar, and sometimes even more striking
evidences of glacial action in widely distinct regions[93]. In
Australia they have been found in New South Wales, Victoria,
South Australia, East Australia and Tasmania; the Dwyka
boulder-conglomerates of South Africa and certain deposits of similar
character discovered by Prof. Derby in Southern Brazil have been
referred to the same period, and their glacial origin has also been
inferred. This widespread distribution of deposits which are generally
contemporaneous, of which the glacial origin may now be taken as
established, is extremely remarkable, and must be taken into careful
consideration by those who put forward theories framed to account for
former climatic changes.
[Footnote 93: The reader will find an excellent account of the
Permo-Carboniferous glacial deposits in a paper by Prof. Edgworth
David, entitled "Evidences of Glacial Action in Australia in
Permo-Carboniferous Time" (_Quart. Journ. Geol. Soc._ Vol. LII. p.
289). In this paper other glacial beds besides those of Australia are
noticed.]
_The Flora and Fauna._ The flora of the Permo-Carboniferous beds has
caused as much discussion as the question concerning the origin of the
boulder-deposits. In the southern hemisphere, the Permo-Carboniferous
rocks of those countries which have yielded boulder-beds also contain
remains of a flora which is now known as the _Glossopteris_ flora,
from the prevailing genus, which is associated with other genera, such
as _Gangamopteris_. These fossils appear to be ferns, though their
modern allies have not been indicated with certainty; associated with
them are rare cycads and conifers. The _Glossopteris_ flora is
markedly contrasted with the Coal-Measure flora of the northern
hemisphere with its giant lycopods. Moreover _Glossopteris_ appears in
the northern hemisphere in rocks of later date than the
Permo-Carboniferous period. It has been suggested that the
_Glossopteris_ flora originated in a continent in the southern
hemisphere, on which the boulder beds were also formed in isolated
water areas, and that some of the forms migrated northwards. To this
continent the name Gondwanaland has been applied by Prof. Suess, from
the _Gondwana_ series of the Permo-Carboniferous rocks of India, in
which the _Glossopteris_ flora is found, and it has also been
maintained that the southern _Glossopteris_ flora was contemporaneous
with the northern flora of ordinary Coal-measure type, though whether
this was so to any extent remains to be proved, for the beds
containing the _Glossopteris_ flora are distinctly newer than any
which have furnished a typical northern Coal-measure flora. In
any case, the change of floras between Coal Measure and
Permo-Carboniferous times is very marked, and when taken in connexion
with the widespread glacial deposits, is one of the most striking
phenomena displayed by the rocks of the stratified column[94].
[Footnote 94: For an account of the Glossopteris flora and its
geological relations, consult Seward, A. C., _Science Progress_,
January, 1897, p. 178.]
The fauna has already been noticed. It consists of brachiopods, some
of which are of peculiar genera. The general similarity of the faunas
in regions so remote as Spitsbergen, the Ural Mountains, India, and
New South Wales, indicates an extensive sea during the period. It can
hardly be supposed that the fauna of Permo-Carboniferous times has
been completely described, for the fossils of one or two areas only
have been made known to us with any degree of fulness, and when the
Permo-Carboniferous and marine Permian faunas are as well known as
those of Triassic times (and the latter have only been fully described
very recently) there is no doubt that the important break which was at
one time supposed to exist between Palæozoic and Mesozoic faunas will
be filled in satisfactorily[95].
[Footnote 95: The Permo-Carboniferous beds are described in Messrs
Medlicott and Blanford's _Geology of India_, second edition (edited by
Mr R. D. Oldham), and figures of some of the important fossils given
therein. For fuller information the reader should refer to Waagen's
account of the Salt Range Fossils and Feistmantel's description of the
plants in the _Memoirs of the Geological Survey of India_.]