GEOLOGICAL SUEVEY OF THE TERRITORIES. 97 



nized Europe by migration ; and that when their connection with their mother country 

 was severed they were overpowered and exterminated by the present flora of Europe, 

 which, as Professor Gray has shown, is mainly of North Asiatic origin. 



The fact to which reference has just been made, viz, the occurrence of Onoclea sensi- 

 tilis on the Island of Mull, off the west coast of Scotland, while it has not been found 

 in the tertiary beds of other parts of Europe, is indicative, so far as it goes, not only 

 of an American connection during the miocene period, but of an American origin for 

 that species; and so, by inference, of the other genera and species common to the two 

 continents during that epoch. 



If this inference should be confirmed by future observations, we should then see how 

 the eocene tropical or subtropical flora of Europe was crowded off the stage by the 

 temperate flora of the miocene, which latter, accompanying a depression of tempera- 

 ture, had migrated from America, while the eocene flora retreated south and east, and 

 is now represented by the living Indo-Australian flora — characterized by its Halcew, 

 Dynandroe, Ucalypti, &c, &c, which form so conspicuous an element in the eocene flora 

 of Europe. This theory would account for the presence of these tropical forms in 

 the lower miocene of Europe, while, so far as yet observed, they are entirely absent 

 from the miocene flora of America. In Europe a few of the eocene forms lingered be- 

 hind in the grand exodus of that flora, and mingled with the more boreal and occi- 

 dental barbarians by which the country was overrun, while in America these which 

 we now call Asiatic forms never had an existence. 



That this bridge between America and Europe was in a temperate climate is proved 

 by the character of the plants which passed over it. On referring to a terrestrial globe 

 it will be seen that by way of Greenland, Iceland, and the Hebrides, there are no very 

 wide gaps to be spanned ; but a connection by that route would carry us so far into the 

 Arctic zone that none of the plants which we suppose to have made that journey could 

 have withstood the cold if the climate had been the same as at present. We have con- 

 clusive evidence, however, that it was not so, for on McKenzie's River, Disco Island, 

 on Iceland and the Island of Mull, we have, in the recurrence of parts of the very flora 

 under consideration, proof, not only of a warmer climate at the far north during the 

 miocene epoch, but that a part of the plants which formed the miocene flora of Europe 

 actually did travel that road ; at least, that they visited all these localities, and, in the 

 buried remains of generations which were never to see the promised land, left us imper- 

 ishable records of the reality of this migration. 



That we cannot, without further study, assign a cause for this great change of climate 

 in the northern part of our continent, is no proof against its existence, for the facta 

 still remain ; the cause of the phenomena is simply a thing to be learned. Several pos- 

 sible causes might be mentioned, but of those which suggest themselves, the deflection 

 of the Gulf Stream seems to me the most natural, simple, and best to account for an 

 elevation of the temperature of Greenland, Iceland, &c. Whether this cause would be 

 sufficient to account for all the phenomena is at least doubtful. A diminution of the 

 land surface at the north, if it could be proved, would help to solve the enigma. Prob- 

 ably several causes conspired to produce this effect, but they were apparently local, or 

 at least terrestrial, as a cosmical cause, producing a general elevation of temperature 

 on the earth's surface, would have given us a tropical flora on the Upper Missouri, 

 whereas we find in the miocene flora there, as yet, really no tropical plants. 



There is one other basin near the sources of the Missouri Eiver which 

 has already yielded many fossils of great interest, but which seems to be 

 isolated from the others. This is what 1 have called the Judith basin, 

 and inasmuch as it seems to be one of the ancient lake deposits, and 

 characterized by a peculiar group of organic remains, I will designate 

 the strata as the Judith Group. / The sediments do not differ materially 

 from those of the Fort Union Group, and they contain impure beds of 

 lignite, fresh water mollusca, and a few leaves of deciduous trees. But 

 the most remarkable feature of this group is the number and variety of 

 the curious reptilian remains, of which we have only yet caught a 

 glimpse. There is probably no portion of the West that furnishes such 

 a harvest of fossil remains and instructive geological facts as the coun- 

 try bordering on the Missouri Eiver, from the mouth of the Yellowstone 

 to the foot of the mountains above the great falls of the Missouri; and 

 as this country is reserved for examination the coming season, 1 

 will leave the obscurity which now invests it to be cleared in the next 

 annual report. 



All the groups of rocks now known to occur in the Northwest are well 

 shown along the flanks and among the foot-hills of the mountains. The 

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