76 FOREST DISTRIBUTIOX 



leaved, deciduous vegetation, is dominant, as a rule, only along 

 the bottom lands and stream banks. It is hardly too much to say 

 that in these places nine-tenths of the forest consists of the spe- 

 cies of Populiis, Salix, Alnus and Betida. On the western slope 

 Popiilus irichocarpa is the only large eottonwood of the river 

 bottoms. It is usually accompanied by Alnus tenuifolia and 

 Betula fontinalis. These three species are found along every 

 stream on the western slope. Along with these, usual or fre- 

 quent, are Popiilus inmuloidts. Salix cordata, S. fluviatihs, 

 Cornus stolonifera, etc. 



On the eastern slope and the plains the bottomlands are 

 commonly covered with Popiiius Sargcntii. accompanied more or 

 less freciuently by P. tremuloidts, Salix fluviatilis, Betula fon- 

 tinalis, AmelancMer alnifolia, Acer Xegundo, Lepargyraea ar- 

 gentea, etc. 



As to the upper limit of tree growth there seems to be no 

 clearly defined timber line except such as may be due to the 

 desiccating influence of wind. Only the crests are usually bare 

 or occupied by stunted pines, spruces or firs and this way oc- 

 cur at relatively low altitudes of 6000-7000 feet, while on neigh- 

 boring mountains of greater elevation forests may be found ex- 

 tending much higher up the slopes. Other factors, like the steep- 

 ness and ruggedness of the land have much to do with the matter 

 of forest occupation both as to moisture and as to foothold. 

 Consequently high and pinnacled crests like the soiithem peaks 

 of the ^Mission Range, the Bitter Roots from St. Hilary 's Peak 

 to the Xez Perce Pass, the precipitous declivities of the main 

 range of the Ruckit^s from Glacier Park to Lewis and Clark 

 Pass, the high summits south of Georgetown, all are barren, due 

 in part to their elevated exposure and to the inhospitable charac- 

 ter of their surface. Elsewhere the high elevations of rounded 

 contour are forested sometimes by pines, firs or spruces, some- 

 times by an open stand of Rocky ^lountain juniper (J. scopu- 

 loriini) as shown near the ^lullan Pass and the Continental Di- 

 vide southeast of Butte. 



The principal trees in the forests west of the Di%'ide are 

 Pinus pondrrosa, P. contorta and P. alhicaulis, Larix occidentalism 

 Pseudotsuga taxifolia. Picca Engelmannii. Abic:< lasiocarpa, and 

 Junipenis scopulorutn. Locally Pinus monticola, Tsuga luttro- 



