180 THE PEINCIPLES INVOLVED IN ROCK-WEATHEEING 



minute subdivision as to remain a long time in suspension, 

 and give the water a milky appearance for several miles. 

 I. C. Eussell has described^ the Tuolumne Eiver, issuing from 

 the foot of the Lyell Glacier in the Sierras of California, as 

 turbid with silt w^hich has been ground by the moving ice. 



At the foot of the Dana Glacier there is a small lakelet 

 whose waters are of a peculiar greenish yellow color from 

 the silt held in suspension, and which, when submitted to 

 microscopic examination, is found to be made up of fresh 

 angular fragments of various silicate minerals of all sizes from 

 0.35 mm. in diameter down to impalpable silt. 



4. ACTIOH OP PLANTS AND ANIMALS 



Both plants and animals aid to some extent in the work of 

 rock disintegration. Plants are also important factors in pro- 

 moting sedimentation, while burrowing insects and animals may 

 exert an important influence upon the texture of soils and in 

 bringing about a more general admixture by transferring to the 

 surface that which is below. 



The lower forms of plant life, — the lichens and mosses, — 

 growing upon the hard, bare face of rocky ledges send their 

 minute rootlets into everV crack and crevice, seeking not merely 

 foot-hold, but food as well. 



Slight as is the action, it aids in disintegration. The plants 

 die, and others grow upon their ruins. There accumulates thus, 

 it may be with extreme slowness, a thin film of humus, which 

 serves not merely to retain the moisture of rains and thus bring 

 the rock under the influence of chemical action, but supplies at 

 the same time small quantities of the organic acids to which 

 reference has already been made. These act both as sol- 

 vents and deoxidizing agents. As time goes on, sufficient 

 soil gathers for other, larger and higher types of life, which exert 

 still more potent influences. It may be the rock is in a jointed 

 condition. Into these joints each herb, shrub, or sapling pushes 

 down its roots, which, in simple virtue of their gain in bulk, day 

 by day, serve to enlarge the rifts and furnish thereby more ready 

 access for water, and the wash of rains, to still further augment 

 disintegration. The depth to which such roots may penetrate has 

 often been noted, varying, as is to be expected, with the nature 



^5th Ann. Bep. U. S. Geol. Survey, 1883-84. 



