THREATENED ARIDITY ON THE PACIFIC SLOPE 165 



into the areas we term semi-arid is to be expected, as they are 

 but a step removed, but it is rather surprising to find them in 

 the midst of subhumid conditions ; yet such is exactly the 

 case. Along the eastern base of the Cascades many of the south 

 and east-facing slopes are distinctly arid, though surrounded by 

 and adjoining decidedly subhumid regions. Similar conditions 

 are encountered on the east, south, and west slopes of the Powder 

 River mountains, on the plateau areas between the Clearwater and 

 the Salmon rivers, in Idaho, and even in scattered localities north 

 of the Snake, among the terminations of the western spurs of the 

 Bitter Roots. Crossing the Bitter Roots and entering the basins 

 and plateaus on the west slope of the main range of the Rocky 

 mountains, we once more meet these extensions of arid con- 

 ditions projecting into the subhumid regions. They are very 

 well marked in the region of the Blackfoot basins, where they 

 cross the main range and connect with the arid upper Missouri 

 plains through the comparatively low passes at the head of the 

 Blackfoot tributaries ; thence stretching westward, they cover 

 large areas of the Clark fork of the Columbia basin, and follow- 

 ing the valley of this stream approach to within 60 or 70 miles 

 of the eastern Washington plains. In the Clark fork watershed 

 these arid extensions are usually bordered by a margin of semi- 

 aridity — their penumbra, as it were — but in many places they 

 join and exist in the midst of the subhumid timbered tracts 

 without any semi-arid transitions. The causes which operate 

 to bring about these apparently erratic and sporadic advances of 

 arid conditions are not very clear. Where they occur in prox- 

 imity to the general body of aridity their presence is easily ex- 

 plained, but we find such tracts covered with herbaceous and 

 shrubby vegetation peculiar to very arid regions in the midst 

 of a forest of yellow pine, or even higher, where the elevation 

 borders on the subalpine. These isolated spots might be com- 

 pared to sparks wafted far in advance of a coming conflagration, 

 each one constituting a nucleus for the further spread of its own 

 peculiar conditions. 



The altitude of the arid tracts varies considerably. At the 

 junction of Snake and Columbia rivers it -amounts to less than 

 150 feet above sea-level. On the southeastern Oregon plateau 

 it rises to fully 6,000 feet on the slopes of various ranges, such 

 as Steins mountains, the ranges to the east of Warner lake, and 

 on unnamed heights between the Paulinas and Malheur lake. 

 Farther north we find the arid tracts at elevations vaiying from 



