164 THREATENED ARIDITY ON THE PACIFIC SLOPE 



to the west of the Owyhee, extending in a westerly and north- 

 erly direction 100 to 120 miles. In eastern Washington the Cas- 

 cade range evidently contributes largely to the aridity which 

 exists on its east slope, and is therefore a local factor; but in 

 eastern Oregon the most arid tracts lie at a distance of 200 

 miles or more east from the Cascades, and owe their origin to 

 the interception of the moisture-laden westerly and southwest- 

 erly winds \>y the Sierra Nevada, and in a lesser degree by other 

 intermediate ranges. Irrespective of local conditions, however, 

 it can be stated as a general proposition, borne out by observed 

 facts, tbat the crest of the advancing wave of aridity in the in- 

 termontane region of southern Idaho, eastern Oregon, and east- 

 ern Washington is traveling from the southeast toward the north 

 and northwest. The tracts termed arid bear*no forests. It is 

 true that narrow fringes of trees skirt many of the rivers or creeks 

 which meander through these areas, but the growth is made pos- 

 sible only by the humid or subhumid soil conditions clue to 

 proximity of streams, and cease at short distances from their 

 banks. 



Looking backward in time, there are abundant proofs that 

 many of the now treeless tracts once bore a forest covering. 

 Silicified wood is found in thousands of localities in the region 

 where no tree growth is now possible, owing to insufficient pre- 

 cipitation, and its occurrence on the surface of these plains, not 

 as transported material, but in place, argues in favor of the h} r - 

 pothesis that the extinction of this forest growth does not date 

 back so very many centuries. The fossil wood, where it occurs 

 on the treeless areas, is found resting directly on the volcanic 

 rocks, indicating that sufficient time has not passed since the 

 forest grew there to change the surface in any perceptible de- 

 gree. The fossil woods referred to consist of remains of oak and 

 probably of pines and junipers, and if not wholly identical with 

 species that now exist in the adjacent regions are very' closel} r 

 related. There are also many localities on these arid tracts where 

 are found fossil plants of Tertiary age imbedded in rock, deeply 

 covered with basalt, but they belong to a period when specific- 

 all} 7 distinct climatic conditions, as compared with those of our 

 age, prevailed in the region, and are not here considered. The 

 areas classed as arid exist in many localities in this intermon- 

 tane region. The most extensive have already been noticed ; but, 

 favored by local conditions, many small lobes from the main body 

 of aridity stretch out on all sides. That they should penetrate 



