Page:EB1911 - Volume 20.djvu/592

Rh Asia; it extended to China and to the Zambesi region of tropical Africa (Map A, I. and II.).

On the other hand, the plant-beds of the Permo-Carboniferous age in South Africa, South America, India and Australia demonstrate

the existence of a widely distributed vegetation which agrees in age with the Upper Carboniferous and Permian vegetation of the north, but differs from it to such an extent as to constitute a distinct flora. We must begin by briefly considering this southern Palaeozoic province if we would trace the Mesozoic floras to their origin, and obtain a connected view of the vegetation of the globe as it existed in late Palaeozoic times and at the beginning of the succeeding era.

In Australia, South America and South Africa a few plants have been found which agree closely with Lower Carboniferous types of the northern hemisphere. In New South Wales, for example, we have such genera as Rhacopteris and Lepidodendron represented by species very similar to those recorded from Lower Carboniferous or Culm rocks in Germany, Austria, England, Spitzbergen, North and South America and elsewhere. It is, in short, clear that the Culm flora, as we know it in the northern hemisphere, existed in the extreme south, and it is probable that during the earlier part of the Carboniferous period the vegetation of the world was uniform in character. We may possibly go a step farther, and assume that the climatic conditions under which the Culm plants of the Arctic regions flourished were not very different from those which prevailed in Europe, Asia, Chile and South Australia. From strata in New South Wales overlying Devonian and Lower Carboniferous rocks certain plants were discovered in the early part of the 19th century which were compared with European Jurassic genera, and for several years it was believed that these plant-beds belonged to the Mesozoic period. These supposed Mesozoic plants include certain genera which are of special interest. Foremost among these is the genus Glossopteris (fig. 1), applied by Brongniart in 1828 to sub-lanceolate or tongue-shaped leaves from India and Australia, which have generally been regarded as the fronds of ferns characterized by a central midrib giving off lateral veins which repeatedly anastomose and form a network, like that in the leaves of Antrophyum, an existing member of the Polypodiaceae. The stems, long known from Australia and India as Vertebraria, have in recent years been proved to be the rhizomes of Glossopteris. It is only recently that undoubted sporangia have been found in clote association with Glossopteris leaves. The genus possessed small broadly oval or triangular leaves in addition to the large fronds like that shown in fig. 1; it was with the smaller leaves that Mr Arber discovered sporangia exhibiting certain points of resemblance to the microsporangia of modern Cycads. We cannot as yet say whether these bodies represent a somewhat unusual type of fern sporangium or whether they are microsporangia; if the latter supposition is correct the plant must have been heterosporous; but we are still without evidence on this point. Associated with Glossopteris occurs another fern, Gangamopteris, usually recognized by the absence of a well marked midrib, though this character does not always afford a satisfactory distinguishing feature. In view of recent discoveries which have demonstrated the Pteridosperm nature of many supposed ferns of Palaeozoic age, we must admit the possibility that the term fern as applied to Glossopteris and Gangamopteris may be incorrect. An Equisetaceous plant, which Brongniart named Phyllotheca in 1828, is another member of the same flora; this type bears a close resemblance to Equisetum in the long internodes and the whorled leaves encircling the nodes, but differs in the looser leaf-sheaths and in the long spreading filiform leaf-segments, as also in the structure of the cones. Phyllotheca has been recognized in Europe in strata of Palaeozoic age, and Professor Zeiller has discovered a new species—P. Rallii—in Upper Carboniferous rocks in Asia Minor (Map A, VII.), which points to a close agreement between this genus and the well-known Palaeozoic Annularia. Phyllotheca occurs also in Jurassic rocks in Italy and in Siberian strata originally described as Jurassic, but which Zeiller has shown are no doubt of Permian age. Some examples of this genus, described by Etheridge from Permo-Carboniferous beds in New South Wales, differ in some respects from the ordinary form, and bear a superficial resemblance to the Equisetaceous genus Cingularia from the Coal Measures of Germany. Other genera characteristic of this southern flora are mentioned later. The extraordinary abundance of Glossopteris in Permo-Carboniferous rocks of Australia, and in strata of the same age in India and South Africa, gave rise to the term “Glossopteris flora” for the assemblage of plants obtained from southern hemisphere rocks overlying beds containing Devonian and Lower Carboniferous fossils. The Glossopteris flora of Australia occurs in certain regions in association with deposits which are now recognized as true boulder-beds, formed during widespread glacial conditions. In India the same flora occurs in a thick series of fresh-water sediments, known as the Lower Gondwana system, including basal boulder-beds like those of Australia. Similar glacial deposits occur also in South America, and members of the Glossopteris flora have been discovered in Brazil and elsewhere. In South Africa, Glossopteris, Gangamopteris and other genera, identical with those from Australia and India, are abundantly represented, and here again, as in India and South America, the plants are found in association with extensive deposits of undoubted glacial origin. To state the case in a few words: there is in South Africa, South America, Australia and India an extensive series of sediments containing Glossopteris, Gangamopteris and other genera, and including beds full of ice-scratched boulders. These strata are homotaxial with Permo-Carboniferous rocks in Europe and North America, as determined by the order of succession of the rocks, and by the occurrence of typical Palaeozoic shells in associated marine deposits. The most important evidence on which this conclusion is based is afforded by the occurrence of European forms of Carboniferous shells in marine strata in New South Wales, which are intercalated between Coal Measures containing members of the Glossopteris flora, and by the discovery of similar shells, many of which are identical with the Australian species, in strata in the north-west of India and in Afghanistan, forming part of a thick series of marine beds known as the Salt Range group. This group of sediments in the extra peninsular area of India includes a basal boulder-bed, referred on convincing evidence to the same geological horizon as the glacial deposits of the Indian peninsula (Talchir boulder-beds), South Africa (Eeca boulder-beds), Australia and Tasmania (Bacchus Marsh boulder-beds, &c.), and South America, which are associated with Glossopteris-bearing strata. We have a flora of wide distribution in South Africa, South America, Borneo, Australia, Tasmania and India which is clearly of Permo-Carboniferous age, but which differs in its composition from the flora of the same age in other parts of the world. This flora appears to have abruptly succeeded an older flora in Australia and elsewhere, which was precisely similar to that of Lower Carboniferous age in the northern hemisphere. The frequent occurrence of ice-formed deposits at the base of the beds in which Glossopteris and other genera make their appearance, almost necessitates the conclusion that the change in the character of the vegetation was connected with a lowering of temperature and the prevalence of glacial conditions over a wide area in India and the southern hemisphere. There can be little doubt that the Indian Lower Gondwana rocks, in which the boulder-beds and the Glossopteris flora occur, must be regarded as belonging to a vast continental area of which remnants are preserved in Australia, South Africa and South America. This continental area has been described as “Gondwana Land,” a tract of enormous extent occupying an area, part of which has since given place to a southern ocean, while detached masses persist as portions of more modern continents, which have enabled us to read in their fossil plants and ice-scratched boulders the records of a lost continent in which the Mesozoic vegetation of the northern hemisphere had its birth. Of the rocks of this southern continent those of the Indian Gondwana system are the richest in fossil plants; the most prominent types recorded from these Permo-Carboniferous strata are Glossopteris, Gangamopteris, species referred to Sphenopteris, Pecopteris, Macrotaeniopteris and other Ferns; Schizoneura (fig. 2) and Phyllotheca among the Equisetales, Naeggerathiopsis and Euryphyllum, probably members of the Cordaitales (q.v. in section I. ); Glossozamites and Pterophyllum among the Cycadales, and various vegetative shoots recalling those of the coniferous genus Voltzia, a well-known Permian and Triassic plant of northern latitudes. The genera Lepidodendron, Sigillaria, Stigmaria, or Calamites, which played so great a share in the vegetation of the same age in the northern hemisphere, have not been recognized among the Palaeozoic forms of India, but examples of Sigillaria, Lepidodendron and Bothrodendron are known to have existed in South Africa in the Permo-Carboniferous era.

We may next inquire what types occur in the Glossopteris flora agreeing more or less closely with members of the rich Permo-Carboniferous vegetation of the north. The genus Sphenophyllum, abundant in the Coal Measures and Permian rocks of Europe and America, is represented by a single species recorded from India, Sphenophyllum speciosum (fig. 3), and a doubtful species from South Africa; Annularia, another common northern genus, is recorded from Australia, and the closely allied Phyllotheca constitutes another link between the two Permo-Carboniferous floras. The genus Cordaites may be compared, and indeed is probably identical with, certain forms recorded from India, South America, South Africa