148 Treeless Areas of the Northwest — Leiherg. 



wholly of semi-carbonized wood, betoken a long continued 

 growth. 



Without attempting to fix the exact geological epoch in 

 which these forests flourished, it can be safely said that the 

 character of the fossil remains indicates very clearly, climatic con- 

 ditions similar to those existing at the present day in portions of 

 our territory 5 degrees or 6 degrees farther south. 



It has been supposed that the country in which we find these 

 fossils, at this time was low and marshy, only here and there 

 rising into low hillocks. Be this as it may, we know that forest 

 growth is closely related to, and dependent upon the amount of 

 the annual rainfall, and the evidence we have proves conclusively 

 the existence in the past of a much more humid climate than the 

 same region now enjoys. 



To the cutting off of this supply of atmospheric humidity, I 

 attribute, more than to any other cause, the extinction of these 

 forests, and the prevention of others from taking their place. 



If w^e travel westward, ^ve find as we progress, the timbered 

 areas increasing in extent, the forests become denser and evincing 

 a more vigorous growth, until on the Pacific slope they reach their 

 maximum in the heaviest and densest forests on the North Am- 

 erican continent. This exuberance of growth is closely graduated 

 according to the amount of the annual precipitation. 

 The rain and snowfall on the Pacific slope is usually accom- 

 panied by winds from the southwest or west. These winds com- 

 ing from the ocean bring up an immense quantity of moisture. 

 Much of it is deposited on the Coast and Cascade ranges, but the 

 broken character of these ranges, and many low passes furnish an 

 avenue of escape for a not inconsiderable portion of the rain 

 clouds to the plains beyond. Comparatively little precipitation 

 takes place here, and the clouds travel almost without interrup- 

 tion eastward, until intercepted by the Bitter Root range. This 

 range, for a distance of 250 miles north and south, stands in the 

 way like a gigantic wall . Its mean altitude is higher than the 

 elevation of the heavy rain clouds; it has but few passes and 

 they not low. In consequence the clouds are banked up against 

 this rocky wall in immense masses, filling the canons and ra- 

 vines with a thick, heavy vapor. Condensation takes place, and 

 great quantities of rain or snow fall on the western slope and 

 summit of this ridge, while but little is deposited on the eastern 



