July lo, 1879] 



NATURE 



257 



THE GENESIS.AND MIGRATIONS OF PLANTS 



SUCH is the title of a paper, in a recent Princettnon Rcviav, 

 by Prof. Dawson, whose intention iu writing has been to 

 place clearly and concisely before his readers the facts, as be 

 interprets them, connected with the fossil floras of the Arctic 

 and North American regions. The necessity to do so became 

 apparent, he states, from the time that Heer described the cre- 

 taceous ^ Vancouver Island flora as miocene, and yet more when 

 the Devonian Bear Island flora was described as carboniferous. 

 The present publication, however, was immediately induced by 

 Saporta's very remarkable essay on the northern origin of plant 

 species and Hooker's latest anniversary address to the Royal 

 Society. 



The Professor commences the present essay by recalling 

 that Asa Gray had, as early as 1867, suggested that the related 

 floras of North America and Eastern Asia had a common 

 northern origin ; and that in 1872 he further developed this 

 theory, embracing in it the work of Heer and Lesquereux on 

 the tertiary floras. 



He then proceeds : — 



" Between i860 and 1870 the writer was engaged in working 

 out all that could be learned of the Devonian plants of Eastern 

 America, the oldest known flora of any richness, and which 

 consists almost exclusively of gigantic, and to us grotesque, 

 representatives of the club mosses, ferns, and mares'-tales, with 

 some trees allied to the cycads and pines. In this pursuit 

 nearly all the more important localities were visited, and access 

 was had to the large collections of Prof. Hall and Prof. 

 Newberry in New York and Ohio, and to those made in the 

 remarkable plant-bearing beds of New Brunswick by Messrs. 

 Matthew and Hartt. In the progress of these researches, which 

 de\-eloped an unexpectedly rich assemblage of species, the 

 northern origin of this old flora seemed to be established by iU 

 earlier culmination in the north-east, in connection with the 

 growth of the American land to the southward, which took 

 place after the great upper silurian subsidence, by elevations 

 beginning in the north while those portions of the continent to 

 the south-west still remained under the sea. 



"When, in 1870, the labours of those ten years were brought 

 before the Royal Society of London in the Bakerian lecture of 

 that year, and in a memoir illustrating no less than one hundred 

 and twenty- five species of plants older than the great carbonif- 

 erous system, these deductions were stated in connection with 

 the conclusions of Hall, Logan, and Dana, as to the distribu- 

 tions of sediment along the north-east side of the American 

 continent, and the anticipation was hazarded that the oldest 

 PaljEozoic floras would be discovered to the north of Newfound- 

 land. Mention was also made of the apparent earlier and more 

 opious birth of the Devonian flora in America than in Europe, 

 a fact which is itself connected with the greater northward 

 extension of this continent." 



The memoir was not published by the Royal Society, and 

 some little disappointment, he says, was thereby occasioned, but 

 it appeared shortly after, although in a less perfect form.- 



In the next place he contends that Heer was in error in 

 supposing that the Bear Island plants are of carboniferous 

 age, and attributes to Heer the responsibility of having led other 

 European geologists to infer that the whole group of beds from 

 the Hamilton to the Chemung were carboniferous, although they 

 underlie the oldest beds of that stage and contain a Devonian 

 fauna. He continues : — 



" In 1872 I addressed a note to the Geological Society of 

 London on the subject of the so-called 'Ursa stage' of Heer 

 -howing that though it contained some forms not known at so 

 (•arly a date in temperate Europe, it was clearly Devonian when 

 t csted by North American standards ; but that in this high latitude, 

 111 which, for reasons stated in the report above referred to, I 

 believed the Devonian plants to have originated, there might be 

 an intermixture of the two floras. But such a mixed group 

 should in that latitude be referred to a lower horizon than if 

 found in temperate regions. In the discussion of these papers, 

 both Sir C. Lyell and Dr. Carruthers argued that the Bear 

 Island flora is truly Devonian. 



" Passing over the comparatively poor flora of the earlier 

 mesozoic, consisting largely of cycads, pines, and ferns, and as 

 yet little known in the Arctic, though represented, according 

 to Heer, by the supposed Jurassic flora of Cape Boheman, we 



' Lesquereux considers this flom to be eocene. 



= " Fossil Plants of the Devonian and Upper Silurian Formations o£ 

 Canada," Pp. 92, twenty plates. (Montreal, 1871.) 



find, especially at Kome and Atane in Greenland, an interesting 

 occurrence of those earliest precursors of the truly modern 

 forms of plants which appear in the Cretaceous, the period of 

 the English Chalk and of the New Jersey greenlands. There 

 are two plant-groups of this age in Greenland ; one, that of 

 Kome, consists almost entirely of ferns, cycads, and pines, and 

 is of decidedly mesozoic aspect. This is called lower creta- 

 ceous. The other, that of Atane, holds remains of many mod- 

 ern temperate genera, as Populus, Myrica, Ficus, Sassafras, and 

 Magnolia. This is regarded as upper cretaceous. Resting 

 upon these upper cretaceous beds, without the intervention of 

 any other formation,' are beds rich in plants of much more 

 modern appearance, and referred by Heer to the miocetie 

 period, a reference warranted by comparison with the tertian 

 plants of Europe, but, as we shall see, not with those of 

 America. Still farther north this so-called miocene assemblage 

 of plants appears in Spitzbergen and Grinnell Land ; but there, 

 owing to the predominence of trees allied to the spraces, it has 

 a decidedly more boreal character than in Greenland, as might 

 be anticipated from its nearer approach to^the pole.'' 



" If now we turn to the cretaceous and tertiary floras of 

 Western America, as described by Lesquereux, Newberry, and 

 others, we find in the lowest cretaceous roc'.s there known — those 

 of the Dakota group — which may be in the lower part of the 

 middle cretaceous, a series of plants ^ essentially similar to 

 those of the so-called upper cretaceous of Greenland. They 

 occur in beds indicating land and fresh-water conditions as pre- 

 valent at the time over great areas of the interior of America. 

 But overlying this plant-bearing formation we have an oceanic 

 limestone (the Niobrara), corresponding in many respects to the 

 European chalk, and extending far north into the British terri- 

 tory,' indicating that the land of the lo«er cretaceous was 

 replaced by a vast Mediterranean Sea, filled with warm water 

 from the equatorial currents, and not invaded by cold waters 

 from the north. This is succeeded by thick upper cretaceous 

 deposits of clay and sandstone, with marine remains, though 

 very sparsely distributed ; and these show that further subsi- 

 dence or denudation in the north had opened a way for the 

 arctic currents, killing out the warm- water animals of the 

 Niobrara group, and filling up the Mediterranean of that period. 

 Of the flora of these upper cretaceous periods, which must 

 have been very long, we know nothing in the interior regions ; 

 but on the coast of British Columbia we have the remarkable 

 cretaceous coal-field of Vancouver's Island, which holds the 

 remains of plants of modern genera, and indeed of almost as 

 modern aspect as those of the so-called miocene of Greenland. 

 They indicate, however, a warmer climate as then prevalent on 

 the Pacific coast, and in this respect correspond with a peculiar 

 transition flora, intermediate between the cretaceous and eocene 

 or earliest tertiary of the interior regions, and which is de- 

 scribed by Lesquereux as the lower liguitic. 



"Immediately above these upper cretaceous beds, we have 

 the great lignite tertiary of the west — the Laramie group of 

 recent American reports — abounding in fossil plants, at one time 

 regarded as miocene, but now known to be lower eocene, 

 though extending upward toward the miocene age.s These 

 beds, with their characteristic plants, have been traced into the 

 British territory north of the forty-ninth parallel, and it has been 

 shown that their fossils are identical with those of the McKenzie 

 River Valley, described by Heer as miocene, and probably also 

 with those of Alaska, referred to the same age.^ Now this truly 

 eocene flora of the temperate and northern parts of America 

 has so many species in common with that called miocene in 

 Greenland, that its identity can scarcely be doubted. These 

 facts have led to scepticism as to the miocene age of the upper 

 plant-bearing beds of Greenland, and more especially Mr. J. 

 Starkie Gardner has ably argued, from comparison with the 

 eocene flora of England and other considerations, that they are 

 really of that earlier date.' 



"In looking at this question, we may fairly assume that no 



' Nordenslciold, " 'Ex^tiinonlo GnxrAani" Geological Magazine, 1872. 



' Yet even here the Bald Cypress (Jaxodium distichum), or a tree nearly 

 allied to it, is found, though this species is now limited to the Southern 

 States. Fielden and De Ranee, Journal of Geological Society, 1878. 



3 Lesquereux. " Report on Cretaceous Flora,*' 



4 G. M. Dawson, " Report on Forty -ninth Parallel." . 



5 Lesquereux's "Tertiary Flora" ; White. On the " Luramie Group ; 

 Stevenson, " Geological Relations of Ligtiitic Groups," Am. Phil, hoc, 



"^"G.'l^f Dawson, " Report on the Geology of the Forty-ninth Parallel," 

 where full details on these points may be found. 

 7 Nature, vol. xviii. p. 124. 



