Jfo.l.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 2~) 



sippi are mainly restricted to places liaving little or uo more rainfall 

 than that mentioned above; also tbat where annual fires have been i)re 

 vented, original prairie surfaces are changing into forests,* and tliat, 

 generally, trees properly planted or raised from seed, with some mirs- 

 ing at the start, are found to thrive along this whole border. 



In view of this, and of the well-known habit of the Indians to burn 

 over the dry vegetation of the plains and prairies in autumn, Ave had 

 thought It most probable " that the line of demarkation between our 

 Avoods and our plains is not where it was drawn by nature"; that "be- 

 tween the ground Avhich receives rain enough for forest and that which 

 receives too little, there must be a debatable border, Aviu^re compara- 

 tiA ely slight causes will turn the balance either way," and Avhere " dif- 

 ference in soil and exposure will tell decisiA^ely," And along this bor- 

 der, annual burnings, for the purpose of increasing and improving buf- 

 falo feed, practiced for hundreds of years by our nomade predecessors, 

 may have had a A^ery marked effect in carrying this woodless district 

 farther eastward than it otherAvise might haA^e reached.t 



Along with this, a more hypothetical cause may be assigned, which, 

 if valid, will help in other explanations. That natural rain-gauge, the 

 Great Salt Lake in Utah, informs us that the rainfall is now increasing 

 over the western border of the region uuder consideration. We know 

 what the maximum height of the water was A^ery long ago ; but we know 

 not the minimum. It is not improbable that this era of increasing moist- 

 ure is of no recent commencement, but has superA'ened on an earlier one 

 of greater dryness than the present, and that this affected the great 

 plains east, as well as the great basin west, of the interi^osed Eocky 

 Mountains. In that case districts may now bear forest, under man's 

 care, which would have been incapable of it before this cycle commenced 

 or had attained the present condition. 



The western portion of these plains is not only drier, but in some 

 parts alkaline, or with other characters of soil uncongenial to forage 

 grasses, especially at the north, where there are only two inches of rain 

 in the three summer and no more in the three winter months. A good 

 deal of the southern part gets about four inches of summer rain, but only 

 half as much in winter. In some parts, accordingly, the characteristic 

 A'egetation of the ultramontane plateau intrudes. Tlie Pulpy Tliorn, 

 ^arcohatus, and its Chenopodeous associates are largely dcA^eloped on the 

 Up])er Missouri waters, accomjDanied by a peculiar Sage-Brush, Arte- 

 nic.sia cana, while the A. trideniafa is rather rarely established on this 

 side of the mountains. 



We have termed this district the region of Buffalo Grass. The grasses 

 form such an inconspicuous and unimi)orttint a feature in the interior 

 arid region that it has not been worth Avhile to mention them, and even 

 on the mountains, except in the alpine region, they are of small account. 



Tide Prof. C. A. White, hi Amer. Jour. Sci., Oct., 1878. 



+ See Forest Geography aud Archcuology, iu Amer. Jour. Sci., 1878, Ser. 3, xvi, 04, — 



