114 THE CANADIAN NATURALIST. [Vol. vi. 



that it is a hydrographic basin, its waters having no outlet to the 

 ocean. The northern part of this area is drained by the Columbia, 

 the southern by the Colorado. Of these the Columbia makes its 

 way into the ocean by the gorge it has cut in the Cascade Moun- 

 tains, through which it flows nearly at the sea level ; while the 

 Colorado reaches the Gulf of California through a series of canons, 

 of which the most important is nearly one thousand miles in length, 

 and from three thousand to six thousand feet in depth. In 

 Volume vi. of the Pacific Railroad Reports, I have de&cribed a 

 portion of the country drained by the Columbia, and have given 

 the facts which led me to assert that the gorge through which it 

 passes the Cascade Mountains has been excavated by its waters; 

 and that previous to the cutting down of this barrier these waters 

 accumulated to form fresh-water lakes, which left deposits at an 

 elevation of more than two thousand feet above the present bed of 

 the Columbia. Similar facts were observed in the country 

 drained by the Klamath and Pit Rivers, and all pointed to the 

 same conclusion. 



In all this region I observed certain peculiarities of geological 

 structure that have been remarked by most of those who have 

 traver&ed the interval between the Sierra Nevada and the Rocky ' 

 Mountains. In the northern and middle portions of the great 

 table lands the general surface is somewhat thickly set by short 

 and isolated mountain ranges, which have been denominated The 

 Lost Mountains. These rise like islands above the level of the 

 plain, and are composed of volcanic or metamorphic rocks. The 

 spaces between those mountains are nearly level, desert surfaces, 

 of which the underlying geological structure is often not easily 

 observed. Toward the north and west, however, wherever we 

 come upon the tributaries of the Columbia, the Klamath or Pit 

 Rivers, we find the plateaus more or less cut by these streams and 

 their substructure revealed. 



Here the underlying rocks are nearly horizontal, and consist of 

 a Variety of deposits varying much in color and consistence. Some 

 are coarse volcanic ash with fragments of pumice and scoria. 

 Others I have in my notes denominated ' concrete,' as they pre- 

 cisely resemble the old Roman cement and are composed of the 

 same materials. In many localities these strata are as fine and 

 white as chalk, and, though containing little or no carbonate of 

 lime, they have been referred to as '* chalk beds" by most travellers 

 who have visited this region. Specimens of this ehalk-like material 



