No.i.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 17 



region or the viciuity of it, with a few exceptions. Primus Peymsylva- 

 nica, Populus halsamlfera, monilifera^ and tremuloides, may be said to 

 come in from the northeast, and only the last extends far into the dis- 

 trict, i he Negundo and Jtmiperus Virginiana, with Fraxinvs viridis, 

 belong- to the Atlantic forest region, and do not penetrate far, unless we 

 count the Oalifornian Negundo as a derivative form. The connection with 

 Pacific forest species is closer; and for the rest they are mainly Mexi- 

 can plateau types, of which the botanical district in question may be 

 regarded as a northern extension. 



2. Characteristics of the herbaceous and shrubby vegetation of the Rocky 

 Mountain forest region. 



It was convenient and, indeed, needful to take the sylva of this region 

 into one view, extending from British Columbia to New Mexico and Ari- 

 zona, and from the Eocky Mountains to the western verge of the Great 

 Basin. But in its northern part the distinction between woodland and 

 woodless country is less marked, and the general botany is comparatively 

 homogeneous throughout the whole latitude, the Atlantic and Pacific for- 

 ests being there in fact confluent. Along the southern border, under 

 very difi'erent conditions and with little and sparse forest, there is an 

 analogous intermingling of the botanical elements, and the general 

 vegetation of these wide-apart extremes is very different. Our personal 

 observations were made on a middle and typical belt, on which the bot- 

 any of the central region under notice is most largely developed and 

 purely exhibited, and where Atlantic and Pacific botany are most widely 

 separated geographically. We shall do well, therefore, to restrict our 

 sketches to this central belt, comprising Colorado and the southern part 

 of Wyoming on the east, Utah in the center, and Nevada at the west. 



And when treating of the vegetation which is fostered by the forest, 

 there is, in fact, only the eastern half of the district to cousider, i. e., the 

 proper Eocky Mountains, the Wahsatch, and the Uintas, which connect 

 these two systems. Far westward, throughout the Great Basin proper, 

 there is not forest enough to impress any botanical character upon the 

 humbler growth, although wherever there is moisture there is a vegeta- 

 tion to correspond. 



As has already been suggested, the timber region is more extensive 

 than the grounds actuallj^ bearing forest. The contraction of the latter 

 to its present limits is, no doubt, largely a consequence of forest fires 

 through a long course of years ; but we suppose that it is also due to 

 an antecedent or accompanjdng stage of increasing desiccation of the 

 country — a stage which, however, had passed its crisis before our ac- 

 quaintance with the region began, the turn being testified to by the in- 

 crease in the height of the water in the Great Salt Lake during the last 

 thirty or forty years. We shall not strain the facts, in any case, if we 

 include in the botany of the forest region, not only the plants which are 

 now sheltered by forest, but those which extend either downward or 

 2 GB 



