6o6 



NATURE 



\Oct, 27, 188 1 



series of basalt rocks with interbedded layers of pala- 

 gonitic tuff, &c. These rocks, so far as my observations 

 go, exactly resemble those of the Fa;r6e Islands. The 

 basalt-rocks are chiefly anamesites, but some are true 

 basalts, while others are dolerites. But in the areas 

 traversed by me I saw none so coarse-grained or so 

 highly porphyritic as those which occur so abundantly in 

 Stromoe, Osterde, and other islands of the Faroes. 

 They form lofty plateaux, deeply gashed with goi'ges, and 

 abruptly truncated, so as to present bold cliffs and 

 precipices to the low grounds at their base, as in 

 the case of the Esja near Reykjavik. Moreover, they 

 appear to be developed chiefly in the maritime districts. 

 Only a glance at these basaltic masses is needed to 

 convince one that they are the mere fragments of what 

 must once have been a most e.Ktensive plateau. The Esja, 

 built up chiefly of comparatively horizontal beds of basalt, 

 tuff, &c., rises to a height of nearly 3000 feet above the 

 low tracts at its base. Nor can there be any doubt thit 

 these beds formerly stretched far away in all directions, 

 and that they have since been removed by the various 

 agents of denudation fro.n the broad undulating low 

 grounds, over which they may still be traced, sometimes 

 continuously for many miles, at other times in sporadic 

 hills and rising grounds which peer above the surface of 

 the recent lavas by which they are surrounded. In short, 

 the Miocene basalt-rocks of Iceland present precisely the 

 same features as the similar rock-masses of the Faeroes. 

 Like the latter they probably formed at one time a wide ele- 

 vated table-land, which has since been cut down and 

 worn away — the lofty walls of the Esja, &c., serving to 

 give us some idea of the enormous erosion that has taken 

 place. Now all this vast erosion had been effected before 

 any of the later lavas, agglomerates, tuffs, &c., in the 

 south-west part of Iceland were erupted. In the region 

 between Hafnarfjord and Krisuvik the lavas have poured 

 through old valleys in the Miocene rocks and spread 

 themselves out over the highly denuded surface of the 

 latter in the opener low grounds. In a word, it is 

 evident that in the south-west part of Iceland a 

 long interval separates the accumulation of the Miocene 

 basalt-series from the eruption of the later volcanic rocks, 

 and I incline to think that the same break in the con- 

 tinuity of volcanic action will be found to hold true for 

 the rest of the island. I believe it will be found that 

 there is no more connection between the display of vol- 

 canic activity in Miocene times and that of the present 

 day in Iceland, than there appears to have been between 

 the volcanic action which manifested itself in Scotland at 

 such widely separated periods as those of the Lower Old 

 Red Sandstone and the Carboniferous. Had there been 

 more or less continuous volcanic activity in Iceland from 

 Miocene times down to the present, we might well be 

 surprised that the later volcanic masses are not much 

 more considerable than they are. If we think of the 

 time required for the removal by denudation of some 

 3000 feet of basalt-rocks, &c., over thousands of square 

 miles, we must be prepared to admit that the volcanic 

 forces cannot have been continuously active. Either they 

 have not been so, or the denuding agents have far 

 surpassed them in energy. 



There is another point which interested me. I found 

 that the whole of the south-west region had been glaciated 

 before the eruption of the later volcanic series. The 

 Miocene basalts are everywhere ice-worn and abraded ; 

 roches vioiitonnces are well-marked, and in many places 

 glacial ruts and striae are conspicuous. Glacial gravels 

 and coarse boulder-clay are li'cewise sprinkled over the 

 surface of the low-lying tracts. Between Reykjavik and 

 Hafnarfjord the glaciation is distinctly from south-east to 

 north-west, and could not have been the result of any 

 mere local glacier. The whole wide tract has been over- 

 flowed by a general }ner de glace. And if this be the case 

 with that part of Iceland which now enjoys the mildest 



climate, we may be sure that the rest of the island must 

 likewise have been enveloped in ice during the Glacial 

 period. In the south-west region all the traces of glaciation 

 Bre strictly confined to the Miocene areas. Nothing of the 

 kind is visible upon any of the later volcanic rocks. These 

 last have flowed over a glaciated surface, for the ice-worn 

 Miocene basalts terminate abruptly at the margins of the 

 wide sheets of black scoriaceous lava, as do also the drift- 

 accumulations of glacial gravels and erratics, while now 

 and again ice-worn knolls of basalt-rock may be seen rising 

 up like islands in the midst of the later lava-fields. Every- 

 where the lavas and their associated agglomerates and tuffs 

 show their original surfaces — the only changes which they 

 have undergone being the result of subaerial weathering. 

 In a word, all the post-Miocene eruptions of the south- 

 west are of later date than the Glacial period. It would 

 be interesting to ascertain whether the same is the case 

 throughout Iceland. As there is every probability that 

 the great break in the continuity of volcanic action, of 

 which 1 have spoken, is not confined to the south-west, 

 but may hold true of the whole island, it seems not un- 

 likely that the conclusions I have formed as to the post- 

 Glacial age of the later volcanic series of the south-west 

 will also be extended to the same series in other districts. 

 In other words, we may yet be compelled to admit that 

 the oldest eruptions of Hecla and her sisters are not only 

 of vastly more recent age than any of the Miocene basalt- 

 rocks, but belong to one of the latest epochs of which 

 y takes cognisance. James Geikie 



THE EVOLUTION OF THE PALEOZOIC 

 VEGETATION 



SOME statements made in Mr. Starkie Gardner's 

 abstract (Nature, vol. xxiv. p. 55S) of the recent 

 work of Saporta and Marion " On the Evolution 

 of the Cryptogams" are so opposed to conclusions 

 at which I have arrived that I can scarcely allow 

 them to pass unchallenged, lest by doing so it may 

 be inferred that I no longer oppose the French school of 

 Carboniferous pahco-botanists on several vital points 

 connected with the interpretation of the Carboniferous 

 flora. But before doing so I may venture to suggest a 

 doubt whether the time has yet arrived for making the 

 attempt to trace the lines of descent of the Palaeozoic 

 flora. It is true that much has been done of late years 

 to extend our knowledge of that flora, but perhaps at 

 the same period our knowledge of the extent of our 

 ignorance has, pari passu, been equally enlarged. We 

 now possess accurate information respecting the structure 

 of many well-known plants, but we have also obtained 

 glimpses of the existence of many obscure but very im- 

 portant organisms which represent factors that cannot be 

 left out of consideration in dealing with the problem of 

 their evolution. Besides this, opinions of experts are 

 widely divergent on some very important questions of 

 interpr.tation affecting the relationship of conspicuous 

 plants whose organisation is understood. So long as 

 experienced palaeontologists are disagreed on the re- 

 lations of the Calamites to the Calamodendra, and of 

 the Lepidodendra to the Sigillarias, a scheme of 

 evolution explaining the development of the Carboni- 

 ferous flora can scarcely be possible. The French school 

 of botanists still believe that what they call Calamites are 

 Equisetaceous Cryptogams, whilst the Calamodendra are 

 Gymnospermous Phanerogams. In like manner they 

 believe the LepidoJendra to be Cryptogams, and as such 

 to be devoid of all e.xogenous growths in the exterior of 

 their stems, whilst they regard all the Lepidodendroid 

 stems that possess such growths as Sigillarise, and rele- 

 gate them also to the Gymnospermic section of the vege- 

 table kingdoai. I am more than ever convinced that 

 these views cannot be sustained, and I think that my 

 memoirs on these subjects, especially Parts IX. and XI., 



