46 COXTEIBUTIONS TO GENERAL GEOLOGY, 1913. 



ported by air currents over vory coasidcriiblc distances of opoii water, none of the types 

 appear to have possessed any niechanisni or device by which they could have spread so widely 

 without a practically continuous land route. It is of course possible that masses of earth 

 covered with livinsj vegetation — iloatinii islands, so called — may have been torn from the land 

 by streams and carried intact for considerable distances into the sea, as similar '"islands" are 

 now transported by the Amazon and other rivers, but the distance over which such masses 

 would be elfective in colonization is relatively short. 



The striking similarity between the Jurassic floras of northwestern North America and 

 eastern Siberia shows that the land connection between these locahties during Jurassic time 

 must have been practically continuous. The known presence of Jurassic beds at ITerendeen 

 Bay on the Aleutian Islands is at least a partial confirmation of the presence of this land 

 britlge. If we may assume that the land adjacent to what is now Bering Sea was the center — 

 or at least an important center — of distribution for the Jurassic floras, then, with conniiuni- 

 cation established between the two continents, the way was open for a wide range over the 

 northern heniisphere. 



The presence of a well-marked Jurassic flora in the Antarctic region goes far toward 

 justifying the presumption of a former land connection to that region, for the present distance 

 is so great as to preclude the reasonable probability that the flora could have reached there 

 otherwise. 



PROBABLE CLIMATIC CONDITIONS. 



With the abundant ilata now available certain tentative conclusions may perhaps be 

 drawn as to the climatic conditions which existed during the life of the Jurassic flora. There 

 is no evidence that this flora was de])aupcrate or pinched as the result of adverse climate. 

 On the contrary, the presence of large and luxuriant ferns, many of them tree ferns, and cycads, 

 and of very abundant and large-sized leaves of Ginkgo, etc., especially in localities far north 

 of the Arctic Circle, affords every evidence that the conditions of growth were favorable 

 From the wide distribution of genera and species it appears that the conditions were also 

 relatively uniform. In other words, there must have been abundant or sufficient moisture 

 and a temperature that would permit the growth of such ]>lants. Knowledge of the probable 

 moisture and temperature refiuirements of a flora now represented only by fossil remains must 

 be gained l)y a study of the nearest related living flora. On this basis it seems safe to assume 

 that the Jurassic flora indicates a moist, warm, pdssibly subtropical climate. 



THE FLORA. 



CoNioPTERis BL'REjENsis (Zalcssky) Scward. 



Plate V, figure 1. 



Coniopteris burejensis (Zalcsiiky) Seward, Jurassic plants from .XinurUind: Com. g^ol. M^m.. new sor., pt. SI, )). 22, 



PI. I, figs. 1-.5; PI. Ill, figs. 18-21. 1912. 

 Dirksonia burcjcnsin Yjalesuky. Notes pal&)phytoIogiques: Com. g6oL IJulI., vol. 22. p. 192, PI. III. tigs. I-I; PI. iV. 



figs. 1-.5, 1904; Knowllon, in Collier, U. S. Geol. Survey Bull. 278. p. 29, 1906. 

 Dicksonia n. sp., Knowllon, in Collier, loc. cit. 

 Cladnphlebis rarcensis Ward, r-^ontaino. in Ward. U. S. Geol. Survey Mon. 48. p. 157. PI. XXXIX. figs. 7. 8, 1905. 



In 1904 Zalessky described a fruiting fern from Amurland under the name Dicksonia 

 hurejensis, at the same time calling attention to its resemblance to Dicksonia coriacea Schenk,' 

 from the Jurassic of Cliiiui, D. coiiciinin TIeer, from Annu'land, etc. In his recent work, cited 

 above, Seward rcdescribed and religured the Zalessk}' spccimensj together with additional 

 material recently procured, under the name Coniopteris burejensis, largely on the ground Ih.it 

 the fossil is undoubtedly congeneric with Coniopteris hymenopliyJloidifi. If it is to be held 

 that Coniopteris is congeneric with Dicksonia, then the former is untenabli*. for it dates from 

 1849, while Dicksonia was founded in ITSS. However, as there is some (l(ini)t as to their 



■Schenk, A., JurassUrhe PSiinzen, in Richthufen\ ilun;i. v.il, 1, p. i')l. I'l, I.I, lit;. -', I'l. Ml. ni;s 



