12 THE BOTANY OF THE ROUTE, 



as far as we could see. But large portions of the ridges west of the river were also bare of 

 trees to an apparent height of about 3,000 feet. Further north we met with none but scattered 

 patches of forest, until reaching the high ridges bounding the vallies of the Methow and Okan- 

 agan rivers, where, as well as along their banks, trees are grouped in beautiful groves, forming 

 a sufficient supply for the population which must in time inhabit these picturesque valleys. 



I have already noticed the prevalence of the forests near the 49th degree along the Columbia, 

 and most of the country thence south to latitude 38° 30' is occupied by them. 



From Fort Colville, southward, to the Spokane river, we found a pleasing country of mixed 

 forests and prairies, with a fertile soil, which is evidently in part due to the intermingling of 

 spurs of the Cascade mountains with those of the Bitter Root range, which appear towards 

 the east well covered with forests on their higher parts. They intercept and precipitate 

 over these northern tracts sufficient moisture to make them highly fertile. 



The lowest points on the Great Plain where trees are found in any abundance are about 2, 000 

 feet above the sea. This most elevated division of the great forest regions of the Territory, 

 covering only the mountain slopes and summits, botanically and zoologically constitutes a south- 

 ward extension of the more northern flora and fauna following the mountain ridges, and thus 

 irregularly interlocking with the third great region of plains. Towards the east and south a 

 dryer climate is found to diminish the extent of forests, until gradually rising higher and 

 higher towards the line of perpetual snow, at length they almost disappear on some of the 

 eastern slopes of the Rocky mountains and on the ranges of southern Oregon and Utah. 



GREAT PLAIN OF THE COLUMBIA. 



Although the great forests west of the Cascade range might most naturally follow in the 

 description of regions after those just mentioned, being allied to them in products and in relation 

 to climatic agencies, I prefer to give here the brief and incomplete observations which I was 

 able to make during our journey over the Great Plain, occupying the central portion of the 

 Territory. 



This region, characterized by an entire absence of trees, occupies an intei'mediate place in 

 elevation between the mountain forests and the lowlands. Though its name gives the impression 

 of a surface i;niformly level, it has (as remarked in my preliminary report) its mountains and 

 valleys, which cannot be separated by any peculiarities of natural products, and must, in a 

 technical sense, be considered as a part of the "plains" region. Thus, on the divide between 

 the Yakima and Pisquouse, I noted that the forests did not appear until near its summit, at 

 nearly 5,000 feet elevation. 



East of Mount Adams the greatest height of the woodless ridges is 3, 000 feet, at the Spokane 

 river about 2, GOO feet, and the lowest point near the centre of the Columbia plain, at Walla- 

 Walla, is 409 feet above Vancouver. Though the caiion of the Columbia is cut down 

 through the elevated plain to the level of 119 feet at the Dalles, the general surface around is 

 much higher, and at the lower points there is little doubt that trees would grow freely if 

 protected from fires, being encouraged by the constant supply of moisture carried through the 

 gap of the mountain by the prevailing strong west winds. 



I may therefore assume an average elevation of from 500 to 2,500 feet for the dry region of 

 the central portion of the Territory, where trees will not grow without artificial irrigation. 



This Great Plain of the Columbia is bounded on the north by an irregular line running between 

 the parallels of 48° and 49°, north of which it is presumed that but few branches of it extend, 



