Treeless Areas of thelXortkwest — Ltiberg. 1 49 



slope, A portion of the packed-up cloud masses is deflected towards 

 the north, and rounding the Bitter Root range is spread out over 

 northern Montana. For this reason the heavy forest-covered 

 area extends near the 49th parallel much farther towards the east 

 than is the case two degrees or three degrees southward. 



One cannot help noticing the sharply drawn line of demarca- 

 tion between the western slope of the above mentioned mountain 

 range and the eastern. The Pacific side being exposed to the 

 moist winds from the ocean, the forests are very similar in charac- 

 ter to those that cover the Coast and Cascade ranges, but on gain- 

 ing the summit and descending on the eastern slope, a change is 

 seen at once. Although the timber is principally composed of the 

 same species that are found on the western slope, yet it is far from 

 retaining the same size and vigorous growth that characterizes 

 that. From this point eastward, the timbered areas dwindle very 

 rapidly in extent and frequency, until the western declivity of the 

 main range of the Rockies is reached. Here, whatever moisture 

 escaped over the Bitter Root range and the evaporation from the 

 country lying between the two ranges is condensed, and in con- 

 sequence we see here and there stretches of fairly well developed 

 timber areas. 



Over the range to the eastward, the arboreal vegetation thins 

 out rapidly, finally ceasing altogether a few hundred miles from 

 the mountains. 



If we now conceive the altitude of the ranges to the west 

 lowered 2,000 to 3,000 feet, a great change would take place. 

 Much of the moisture now deposited on the western slopes would 

 be carried over the Bitter Root range to the eastward, causing a 

 large increase in the annual rain fall, and elevating the mean 

 annual temperature very considerably. 



This was doubtless the existing condition at the time these 

 now petrified forests of the northwest flourished. The Coast, Cas- 

 cade and Rocky Mountain ranges had but recently emerged from 

 the ocean; the Bitter Root, though elevated long anterior, had not 

 yet reached a height sufiicient to offer any obstructions to the un- 

 interrupted flow of atmospheric humidity from the Pacific: besides 

 there certainly existed large bodies of water in the basins between 

 the ranges. Gradually the country was elevated, the marshes, 

 ponds and basins were drained, and the ranges to the westward 

 deflected and cut off the air currents from the Pacific. This* led to 



