ON THE TIMBER LINE OF HIGH MOUNTAINS. 569 



and deep snows would be less frequent; and not sufficient to interfere much with 

 arboreal growth. But the rain would of necessity carry down the earth and dis- 

 integrated rock to lower levels ; and the melting snows, such as there were, would 

 make this downward progress of the soil continuous. In some mountains where 

 the rock was easily broken by frost, as in Colorado and the White Mountains, it 

 would be very difficult for the soil to hold its own against these forces of gravita- 

 tion; but on more solid rock the mass of tree roots protecting the rock, and re- 

 taining the earth matter, would longer hold its own. In the former case with the 

 gradual washing away of the earth the larger trees will have to find a lower level ; 

 the summit condensing more moisture, and having a cooler atmosphere, would 

 form heavier masses of longer enduring snow, and thus keep down from tall 

 growth the younger trees left as the older and larger ones retired. They would 

 have to be low bushes by the absence of earth for vigorous growth, and remain 

 trailing bushes, through the superincumbent and long continued mass of snow. 



We thus see that though a long continued mass of snow has much to do in 

 marking a timber line, that line is precedent to the snowy mass. The primary 

 cause is the gravitation of disintegrated rock — the movement of the hill-top to- 

 ward the sea. From the moment the mountain reaches its highest point it com- 

 mences its downward march. The entire reduction of the highest to a level with 

 the plain is but a question of time. The frost and rain and melting snow will do 

 it all, and this reduction, bringing down not only the earth, but cold-loving plants 

 to warmer levels, must continually change the aspects of vegetation, as well as 

 perpetually vary the timber line. 



In low hills as well as in high mountains the forces of gravitation are also at 

 work. But the sides are seldom so steep as in the loftier hills, — the rains do not 

 gather with such force nor are the melting snows of near the same duration. 

 There are sudden washes, but not the continuous roll of the earth to the bottom. 

 In time they may exhibit the same phenomena of the disappearance of species 

 from their summits as their loftier brethren ; but the centuries here will gather 

 much more slowly to produce a similar effect. 



In conclusion he would say briefly that the "timber line " of high mountain 

 tops results from the washing down of the earth from the higher elevations. — 

 JProceedings of the Academy of Natural Sciences of Philadelphia. 



