GEOGRAPHY OF NORTH AMERICA. 211 



their northern side back to Rawlins, on the Union Pacific 

 Railroad. 



By this survey he has been able, as he believes, to show 

 the identity of the lignitic series of strata east of the Rocky 

 Mountains, in Colorado, with the Fort Union group of the 

 Upper Missouri River, and with the great Laramie group of 

 the Green River basin. The relative age of these beds has 

 been a matter of long dispute among geologists, but the in- 

 vestigations of this year have proved their complete equiva- 

 lence by the discovery not merely of one or two doubtful 

 species common to the strata at any one point, but by an 

 identical molluscan fauna ranging through the whole series 

 in each of these regions. He also finds the plains of demar- 

 cation between any of the Mesozoic and Cenozoic groups, 

 from the Dakota or Lower Cretaceous to the Bridger or 

 Higher Tertiary, inclusive, to be either indefinable or very 

 obscure ; showing that, whatever abrupt changes may have 

 taken place elsewhere during that period, sedimentation was 

 here probably continuous. While each of the groups of 

 either series possesses its own peculiar paleontologic.il char- 

 acteristics, it is also true that certain species pass beyond 

 the recognized boundaries of each within the series. 



While this ends our account of the regular exploring par- 

 ties attached to the survey, it does not complete its activi- 

 ties. Under its auspices several parties have been making 

 special investigations in the field. To gain a general con- 

 ception of the vegetation of the Rocky Mountain region, 

 and of its relations to that of the rest of North America, Sir 

 Joseph Hooker and Dr. Asa Gray examined many parts of 

 Colorado, Wyoming, Utah, Nevada, and California, travelling 

 in all some 9000 miles. The results of their studies will ap- 

 pear in detail through the publications of the survey; but 

 they are already able to state that the vegetation of the mid- 

 dle latitudes of North America resolves itself into three prin- 

 cipal meridional floras, far more diverse than those presented 

 by any similar meridians in the Old World being, in fact, 

 so far as the trees, shrubs, and many genera of herbaceous 

 plants are concerned, absolutely distinct. Each of them is 

 subdivisible into three. The first region comprises the At- 

 lantic slope and Mississippi valley, and is subdivisible into 

 an Atlantic, a Mississippi, and an interposed mountain, or 



