1^6 



NATURE 



[June 19, 189a 



Sigillariae, and other extinct types. Conifers grow now in 

 very severe climates, and only the tree-ferns really indicate 

 warm climatic conditions. At the present day their chief 

 development is in the tropics, and they require, not indeed 

 great heat, but the absence of frost. We do not, however, 

 know that this was equally the case in former ages ; in 

 the Carboniferous period, the highest division of the 

 vegetable kingdom, now so dominant, the flowering 

 plants, were either non-existent, or were sparsely repre- 

 sented only by a i^^ early forms, and it is by no means 

 improbable that these types in their gradual extension 

 have exterminated the tree-ferns in the colder regions to 

 which they formerly extended, and that these latter have 

 lost the power which they once possessed of withstanding 

 frost. 



Another fact that has been adduced to prove the 

 former prevalence of a warm climate, is the great thickness 

 of the beds of coal, which, it was assumed, could only 

 have been formed by a luxuriant vegetation stimu- 

 lated by a high temperature. But this also is incorrect ; 

 remarkably rich plant-growths are to be met with also in 

 countries with very severe climates, and indeed few 

 countries surpass, in this respect, the inhospitable Terra 

 del Fuego, with its impenetrable beech forests. More- 

 over, there is no good ground for the assumption that a 

 luxuriant growth of plants is necessary for the formation 

 of thick beds of fossil fuel. At this present time we 

 know of but one mode in which vegetable remains 

 accumulate in thick beds, and thus exhibit to us the 

 first step of the process of coal formation : this is the 

 formation of peat, which, as is well known, is effected by 

 the most inconspicuous and poorest of plants, viz. certain 

 kinds of mosses. It is not in the towering primaeval 

 forests of India and Brazil, nor the mangrove swamps of 

 tropical coasts, but in the moors of the sub-arctic zone, 

 that plant-remains are now being stored up in a form 

 that, in the course of geological ages, may become con- 

 verted into beds of coal. 



A closer examination of these conditions apprises us of 

 certain important facts. The reason why great masses 

 of vegetable remains do not accumulate in warm 

 countries is that, in the presence of a high temperature 

 the decaying plants decompose too rapidly, and speedily 

 disappear ; it is only in a cold climate that they are 

 preserved ; and we may therefore regard the existence of 

 coal-beds as a proof that at the time of their formation 

 a high temperature did tiot prevail. 



Out of the mass of baseless assumptions, then, this 

 tolerably well-founded fact remains, that an arborescent 

 vegetation of the Carboniferous period presents itself in 

 76° of northern latitude, whereas, at the present day the 

 northern limit of tree-growth nowhere exceeds 72° ; and 

 if we assume that there has been no displacement of the 

 earth's axis of rotation, we must conclude that in these 

 high latitudes the mean temperature of the year was 

 formerly some degrees warmer than at this present time ; 

 in the temperate zone we may infer, with some probabihty, 

 a cool climate with moderate heat in the summer and 

 cold in the winter, and with but little frost : in fact, an 

 insular climate, such as our knowledge of the distribution 

 of land and sea in that age presupposes. 



So far we have regarded only the conditions obtaining 

 in the north temperate zone and the polar regions. 

 These, however, show certain peculiarities of distribution. 

 The greatest coal deposits are all in the temperate zone, 

 and chiefly concentrated in its middle and northern 

 regions. The most northerly of the great deposits of 

 the productive Coal-measures are those of Scotland, the 

 most southerly those on the border of the central plateau 

 of France ; such as lie further north or south are of little 

 importance. In North America, it is true, they extend 

 considerably further south, but none reach to the 30th 

 parallel of latitude ; while, in the north, they extend into 



NO. 1077, VOL. 42] 



British North America. The coal of China occurs in the 

 northern provinces, in Shansi, Shensi, and Honan. 



Thus we find that the greater deposits are restricted to- 

 a zone of variable width, the southern limits of which are 

 between 30° and 45°, the northern between 50° and 60" 

 N. lat. ; beds of true coal of the same age are not indeed 

 entirely wanting outside these limits, but they are rare ; 

 as a rule we meet with only the characteristic plants, and 

 these gradually disappear as we proceed further south. 

 In a few instances they may be traced as far as Northern 

 Africa and the peninsula of Sinai ; but between the 

 tropics the typical flora of the coal formation seems to 

 fail entirely ; not a single instance of their occurrence can 

 be cited ; and their first reappearance seems to be in the 

 southern temperate zone in the coal-fields of Southern 

 Brazil. 



For a long time it was very doubtful what explanation^^ 

 should be given of this phenomenon, whether plant- 

 bearing deposits of this age were altogether wanting in 

 the tropical zone, or whether their development was of 

 so different a character that we had failed to identify 

 them, or finally whether it were due to some other cause. 

 We cannot notice at length the gradual development of 

 our knowledge on this head ; we can only sketch out the 

 final results which have been yielded in the last few years. 

 We know now, that in Southern Africa, in India, and 

 Australia, there are extensive deposits of the same age 

 as our productive Coal-measures, with abundant plant- 

 remains, but that these differ very greatly from the 

 contemporaneous growths of our own region. No trace 

 is found of the forms characteristic of our Coal-measures, 

 no Sigillariae, no Lepidodendrons, no Calamites. Ferns 

 and true Equisetaceae furnish by far the greater part of 

 the flora ; and with these are associated a small number 

 of conifers and Cycads. The commonest and most 

 characteristic form of this flora is the fern genus 

 Glossopteris, and accordingly the whole assemblage of 

 associated plants has been termed the Glossopteris flora. 

 When put in comparison with our European coal flora, so 

 strange does this seem, that no one would venture to 

 think of it as contemporary until it had been established, 

 by evidence admitting of no question, that such is 

 actually the case. 



From this, however, the important result follows that 

 the doctrine of a universal coal flora is altogether false. 

 On the contrary, we find that we have to deal with two 

 very different floral regions, which stand strongly con- 

 trasted. And what makes this contrast especially 

 remarkable, and for a long time hindered its true 

 interpretation, is that the Glossopteris flora of India, 

 Australia, and South Africa is nearly related to the 

 European flora of a much later period, viz. the Trias. 



But the most striking fact connected with this flora is 

 that its first appearance, whether in South Africa, India,, 

 or Australia, is associated with deposits of fine argilla- 

 ceous sand, with numerous stony fragments varying in 

 size from small pebbles to gigantic blocks of many 

 hundredweight, irregularly embedded ; they consist for 

 the most part of rocks that do not occur anywhere in the 

 neighbourhood, and must therefore have been transported 

 from a distance, and moreover some among them are 

 scored and scratched. These phenomena, which manifest 

 themselves in three far-distant locahties, and according 

 to the latest intelligence seem to recur also in Brazil^ 

 bear such striking evidence of the agency of ice in the 

 formation of these deposits, that any doubt on this head 

 seems scarcely any longer admissible, however much it 

 may startle us to find great ice-masses and floating ice- 

 bergs at the time of the coal formation in regions so far 

 from the poles. 



From the facts we have recounted, bearing on the climate 

 of the Coal-measure period, it is abundantly manifest 

 that everything runs counter to the assumption of a uniforra 



