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430 FLORA OF THE LAKAMIK (iltori'. 



yet, SO far as any accumulatt'd evidence shows, those luamuialia were 

 not preceded in the Laramie period by any related forms. Such sud- 

 denness of introduction makes it almost certain that it was caused by 

 the removal of some jihysical barrier, so that ground which was before 

 potentially Tertiary became so by actual faunal occupancy. In other 

 words, it seems certain that those Tertiary mammalian types were 

 evolved in some other region before the close of the Laramie period, 

 where they existed contemporaneously with at least the later Laramie 

 Dinosaurians of Cretaceous types, and that the barrier which separated 

 tlie laiHue was removed by some one of the various movements con- 

 nected with the evolution of the continent. The climate and other 

 jihysical coTiditions which were essential to the existence of the Dino- 

 saurians of the Laramie period having evidently been continued into 

 the Tertiary epochs that are represented by the Wahsatch, Green River, 

 and Bridger groups, they might doubtless have continued their exist- 

 ence through those epochs as well as through the Laramie period, buffer 

 the irruption of the mammalian horde, to Which they probably soon 

 succumbed in an unequal struggle for existence." 



From the above extracts it will at once be seen that Dr. White had 

 now succeeded in raising this discussion from the comparatively trivial 

 question as to the name which should be given to the age occupied by 

 the Laramie group to one involving not only the manner in which the 

 continent was formed, but also the origin, development, extinction, and 

 succession of the different forms of life which have left in the rocks a 

 trace of their former presence as constituting its inhabitants. The 

 consideiations last urged have an especial interest from the i)oiut of view 

 of vegetable paleontology, which presents a close parallel, though at a 

 considerably lower horizon. 



In the next annual report Dr. White goes over the same ground 

 and sets forth his views anew, supported by fresh facts. In fixing the 

 boundaries of the Laram ie sea, he says (p. 49) : " The geographical limits 

 of the Laramie group are not yet fully known, but strata bearing its 

 characteristic invertebrate fossils have been found at various localities 

 within a great area, whose northern limit is within the British Posses- 

 sions and whose southern limit is not further north than Southern Utah 

 and Northern New Mexico. Its western limit, so far as known, may be 

 stated as approximately ujion the meridian of the Wahsatch range of 

 mountains, but extending as far to the southwestward as the southwest 

 coiner of Utah, and its eastern limit is far out on the great plains, east 

 of the Rocky Mountains, where it is covered from view by late forma- 

 tions and the prevailing debrin of the plains. These limiis indicate for 

 ' J, the ancient Laramie sea a length of about one thousand miles north 



■ ■' and south, and a maximum width of not less than five hundred miles. 



Its real dimensions were no doubt greater than those here indicated, es- 

 ^y^' peeially its length ; and we may safely assume that this great brackish- 



water sea had an area of not less than fifty thousand square miles." 



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