856 STUDIES FOR STUDENTS 



Aside from this physical action plants promote disintegra- 

 tion by keeping the surface of rocks continually moist, and 

 through their decay by supplying the complex series of com- 

 pounds commonly called humic, ulmic, crenic and apocrenic 

 acids to which reference has already been made. These act 

 both as deoxidizing and solution agents. 



"There is reason to believe that in the decomposition effected 

 by meteoric waters and usually attributed mainly to carbonic 

 acid, the initial stages of attack are due to the powerful solvent 

 capacities of the humus acids. Owing, however, to the facility 

 with which these acids pass into higher states of oxidation, it is 

 chiefly as carbonates that the results of their action are carried 

 down into deeper parts of the crust or brought up to the sur- 

 face. Although CO3 is no doubt the final condition into which 

 these unstable organic acids pass, yet during their existence 

 they attack not merely alkalies and alkaline earth, but even dis- 

 solve silica." ^ 



It is stated by Storer that "on the tops of the higher hills of 

 New Hampshire, and on the coast of Maine also, a cold, sour 

 black earth will often he noticed at the surface of the ground, 

 immediately beneath which is sometimes a layer of remarkably 

 white earth. The whiteness is due to the solvent action of acids 

 that soak out from the black humus, and which leach out from the 

 underlying clay and sand the oxides of iron that formerly col- 

 ored them, leaving only the insoluble pure clay or sand." 



H. Carrington Bolton has shown that very many minerals are 

 decomposed by the action of cold citric acid for a more or less pro- 

 longed period, the zeolites and other hydrous silicates being espe- 

 cially susceptible. Such tests have a peculiar significance when 

 we consider that the roots of growing plants secrete an acid sap 



^ Geikie, Text-book of Geology, 3d ed., p. 472. The writer was shown not long 

 since, a very practical illustration of the remarkable corrosive power of organic acids. 

 A highly ornate French clock with case of black marble was packed for storage in 

 excelsior which was a trifle damp. The clock remained in storage from the last of 

 May until about the first of October. When the packing material was removed, the 

 marble was found to be so corroded as to need rehoning and polishing. The rough- 

 ness could be easily felt by passing the finger over the surface, and long lusterless 

 lines indicating the contact of excelsior fibers traversed the surface in every direction. 



