The Principles of Stratigraphical Geology

by J. E. Marr

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.

[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].