WEATHERING 



19 



done by the wind in wearing down solid rocks and coarse soil mate- 

 rials into dust. The impact of sand particles against rocks and 

 against each other gradually wears them down into fine materials 

 ( Fig. 11). The largest number of particles are moved near the sur- 

 face of the ground, hence the greatest amount of abrasion will take 

 place there. A boulder will be worn away slowly on the windward 

 side at the base until it topples over, and the process will then l>e 

 repeated until it is entirely destroyed. Along shores the glass in 

 windows of houses is sometimes worn through by the impact of 

 sand particles, and an instance is 

 given by Merrill 3 where the glass in 

 a lighthouse was ruined during a sin- 

 gle storm. Sand blasts are used to 

 produce ground glass. The natural 

 monuments and " mushroom " rocks 

 in the West owe their origin largely 

 to the work of the wind. 



(g) Plants. The mechanical 

 action of plants is shown by the 

 growth of roots in crevices or fissures 

 of rocks and the prying apart of great 

 masses, thus giving other agencies 

 an opportunity for effective work. 

 The force exerted by mushrooms or 

 toadstools is sometimes sufficient to 

 raise blocks of stone, while cement 

 walks are frequently ruined by the 

 lifting action of roots of trees grow- 



*. . ,... , 4 



lllg adjacent (I'lg. 12). Hllgard* 

 i ii i <t * i i 



makes this statement, Actual meas- 

 urement has shown the force with which the root. e.g.. of the garden 

 pea penetrates, to be equal to from seven to ten atmospheres per 

 square inch." 



II. CIIKMICAI, ACKNC1KS. 



(a) Acids. The atmosphere in all localities contains more or 

 less acid gases, which in combination with the moisture of the air 

 form acids that are brought down with the rain. These acids are 

 much more abundant in the vicinity of manufacturing plants, smel- 

 ters. and large cities where they are produced, largely by the burning 

 of coal. Sulfuric acid is probably the most common of these and 



Fio. 12. The roota of trw 

 wedges for prying rocks apart. 



l)orl, U. S. Geol. Survey.) 



