462 BOTANICAL GEOGRAPHY. 



The open prairie-steppe beyond the Rocky Mountains is covered generally 

 with shrubs of Artemisia, between which, however, cattle everywhere find 

 food in nutritive grasses. Purshia tridentata, one of the Spiraeacese, which 

 frequently accompanies the Artemisia, is a shrub peculiar to this part. The 

 nutritive plants used there by the Indian hunters in cases of necessity, cor- 

 responding to the Psoralea esculenta on the Missouri, consist of Valeriana 

 edulis (Tobacco-root), Cirsium mrginianum, a species of Anethum (Yampeh), 

 and Kamassa (Kamas), Fr. indescr. The bank-forests of the cotton-wood 

 (Populus) are not met with until we arrive at the lower regions : they appear 

 to be entirely wanting on the upper terrace. At the bifurcation of the 

 Oregon, where the prairie terminates (101 W. long.), the wooded promon- 

 tories of the western chain of high mountains commence, which may be 

 compared to the Rocky Mountains in extent, and projecting everywhere 

 above the snow-limit, probably exceed them in height. Being a continua- 

 tion of the Californian Andes, it is called, in Upper California, Sierra Nevada, 

 in Oregon, the Blue Mountains, and the Cascade-chain, where, on the south 

 side of the united rivers, near Fort Vancouver, it rises into high snow-moun- 

 tains, as at Mount Hood. At Oregon, the forests of this high moun- 

 tain-chain (explored between 2700' and 3800'), which are only interrupted 

 by the most splendid meadow-slopes, consist of birch, but above all of 

 various Coniferous trees remarkable for their enormous dimensions, which 

 are such as are not met with in any other part of the globe. The larches 

 were sometimes 200' high (p. 182), the firs were of the same height, with 

 stems T in diameter ; in the former, the unbranched stem beneath the crown 

 was sometimes 100' in length. "White spruces which gave off branches 

 down to the root appeared nevertheless to measure 180', perhaps 200'. 

 The Cascade-chain separates the mild climate of the western coast of the 

 Oregon district from the dry prairies equally as definitely, but in an inverse 

 direction, as the Peruvian Andes do the west desert coast-region from the 

 more humid higldands. This meridional mountain-chain, which intersects 

 the River Columbia about twenty-five or thirty miles from its mouth, receives 

 the mists and rain which are driven over to it from the Pacific Ocean, but 

 which do not penetrate to the clear sky of the steppe. At the rapids of the 

 Columbia, the " Dalles" within the mountain-line, the rainy season is already 

 unknown, which on the coast denotes the winter, and this season is only 

 recognisable there (45 N. lat.) by a slight fall of snow, which scarcely lasts 

 on the ground for two months. The cause of the winter rainy season at the 

 mouth of the Oregon, where west winds predominate, appears to me to 

 depend simply upon the fact, that in the summer the sea, whilst in the winter 

 the continent, is the coldest, so that during the latter period of the year, the 

 humid winds from the sea must quickly lose their moisture in passing over 

 the coast-district. But the steppe lying behind the mountains is a highland ; 

 as such it exceeds the coast in warmth and dryness, and cannot, therefore, 



