BOTANY OF THE ROUTE. 15 



occupies a zone along the eastern side of the mountains between the heights of 2,500 and 

 5,000 feet at the Columbia river, and becoming lower as we go northward, until at fort Colville 

 and on the Okanagan river, at latitude 49, it extends quite down to the level of the rivers, 

 occupying all the surface except some small prairies in the valleys. 



At the mountain gaps of the Columbia and Yakima rivers trees also extend further down 

 along the streams, but in small numbers. The well marked and usually abrupt lower limit of 

 these forests evidently corresponds to the degree of moisture derived either from the rains of 

 the mountain summits, or from the rivers. The moist winds from the ocean, intercepted by 

 the highest ridges, pass through the two gaps above mentioned, and to a small extent favor the 

 growth of trees lower down. Doubtless the cessation of fires on the dry plains will be 

 followed by a further increase of forests in such places. 



North of latitude 48, the country being generally hilly, is better supplied with rains; and 

 on the northern slopes of the hills I observed dense forests, while frequently their soutJiern 

 exposures were bare, showing the direction of the prevailing winds and consequent moisture. 

 This fact was also noticed by others among the western spurs of the Rocky mountains. The 

 same effect is well marked southward on Cape Mendocino, in California; while thirty miles 

 south of San Francisco trees almost entirely of a distinct and southern group grow chiefly on 

 the southern slopes, indicating a corresponding difference in the direction of the moist winds. 



The dry season was already far progressed, and I found, therefore, but few plants in a con 

 dition for preservation, though those collected happened to be of peculiar interest. (See Phce- 

 lipoea comosa, Erigeron Douglasii, Poeonia Brownii, Spraguea umbellata, Acer glabrum, &c.) The 

 entire vegetation seems to belong to the Rocky mountain group much more than that of the 

 western slopes, although several of the plants are, as far as- known, peculiar to this range and 

 the Sierra Nevada. 



A corresponding group of animals also first appeared there, such as the coyote, badger, and 

 Say s striped squirrel; but large game continued very scarce, and the season was unfavorable 

 for birds, which seemed to have almost all deserted these forests during August. 



To complete their description, I extract from my journal the notes on these forests as they 

 appeared further north, and at later seasons. 



On September 13 I rode from the camp on the Yakima about fifteen miles up its valley, 

 and found the forest commencing about six miles up, at an elevation of about 2, 200 feet, as 

 abruptly as where we left it before, with exactly similar vegetation and the same dreary absence 

 of animal life. 



On the 20th the expedition crossed the ridge separating the waters of the Yakima and 

 Pisquouse rivers. There, 5, 750 feet above the sea, we found a scattered belt of forest, in 

 which the larch, (L. OCCIDENTALS, Nuttal) appeared, of great size, and about equal in abund 

 ance with the pines. This magnificent tree sometimes excels the latter in size, and its feathery 

 foliage, just beginning to fade yellow, gave it a beautiful appearance in contrast to the deep 

 green forest around it. With these were a few scattered spruces of several species, which I 

 could not well determine, finding no cones. 



From the summit of this ridge we had a panoramic view of a vast extent of country on all 

 sides of us. Towards the west the numberless irregular peaks of the Cascade range looked 

 like the confused waves of a rough sea. Below their highest snow-capped peaks the belt of 

 forest could be plainly seen extending down on spurs to the Ot)lumbia, but crossing it only at 

 a far distant point near the northern boundary of the &quot; Great Plain,&quot; which extended eastward 



