46 BIOLOGICAL LECTURES. 



of the warm wind itself is often very injurious. The presence 

 of the chinook results in the elimination of the species which 

 are particularly susceptible to the warm, dry air from the locali- 

 ties affected. 



A few years since, the leaves of coniferous trees on the lower 

 slopes of Pike's Peak were killed in this way, and the San Fran- 

 cisco Mountains offer marked examples of this action. On 

 the eastern slopes of the principal elevation of this group the 

 coniferous forest reaches an elevation of 3000 to 3500 meters. 

 On these slopes, in the localities which are exactly suited to 

 receive the strongest action of the chinook, are extensive areas 

 of pine and spruce trees which, so far as repeated examination 

 by myself and others shows, were not destroyed by fire or dis- 

 ease, and the supposition is fairly allowable that the chinook 

 was the destructive factor. This supposition is strengthened 

 by the fact that the small hills and volcanic cones which stand 

 directly to the eastward in the path of the winds lose their snow 

 before other places of the same altitude and geological forma- 

 tion elsewhere in the region. 



In addition to the action of the ascending currents of air 

 on the temperature of highlands they often occasion serious 

 disturbances of the moisture conditions in a highly localized 

 manner. 



The ascension of a current of warm air from the bottom of 

 a valley or canon necessitates the loss of some of its heat to 

 furnish the energy for expansion, and the consequent cooling 

 entails the lowering of the dew point. This action may pro- 

 ceed until the point of saturation is reached and precipitation 

 occurs. More generally, however, the current reaches the 

 highlands as an extremely moist wind. The margins of mesas 

 and hilltops adjoining deep valleys are most subject to such 

 moist winds, and as a consequence these localities offer suitable 

 conditions for moisture-loving species which thus may occur in 

 great abundance as oases in an arid region. As the moisture- 

 laden current traverses the high plain, it soon becomes heated 

 to the temperature of the air through which it is flowing, and 

 its relative humidity is decreased. 



The moisture influence of ascending currents is strikingly 



