832 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Chemical agents produce such changes in the rock as may, and most 

 frequently do, result in its partial or complete decomposition. This 

 decomposition is the destruction of original and the consequent forma- 

 tion of new compounds. Very often it is accompanied by an increase 

 of volume, whereby the original molecular cohesion is disturbed. On 

 the other hand, it may result in the removal of certain constituents, 

 thus producing an effect directly inverse to the former. By either of 

 these processes, the mass is disturbed in such a manner as to render 

 it less impregnable to the attacks of physical erosives. Although we 

 cannot have, therefore, a truly chemical erosion in all instances, we are 

 justified in using the term, because the chemical action is the immediate 

 means by virtue of which the mechanical work may be accomplished. 



Most prominent among the chemical agents facilitating mechanical 

 erosion are water, either pure or charged with various gases, and grow- 

 ing vegetation. Minerals like feldspar, anhydrite, and others absorb 

 water, and are changed into caolinite and gypsum respectively. Both 

 of these secondary products are less able to withstand erosion than 

 the original compounds. This represents the case where changes of 

 chemical composition prepare the material in such a manner as to 

 offer the least resistance to physical erosives. Water charged with 

 gases, more particularly carbonic acid gas, will dissolve certain com- 

 pounds readily and carry away portions thereof in solution. Hot and 

 cold water, pure, will act in the same manner, but to a less degree. 

 Growing vegetation will chemically absorb certain ingredients of rocks 

 upon which its roots may be resting, thus either directly removing 

 small quantities of the material or changing its chemical composi- 

 tion. This erosive action by vegetation becomes insignificant, how- 

 ever, when compared with the far superior physical force growing 

 plants exhibit. Gases contained in the atmosphere have some effect 

 upon rocks of varying constitution, but frequently one that tends 

 rather to preserve than to destroy the material acted upon. Oxyda- 

 tion is the most widely distributed result of such influence. 



Most prominent among the agents of physical or mechanical erosion 

 is the action of water, wind, and growing vegetation. Again, we find 

 that by vegetation the subsequent absolute removal of material is pre- 

 pared. The growth of roots in minute crevices of rocks may frequently 

 result in a disruption of the cohesion, thus either directly removing 

 a fragment or placing it into such a position as to make its removal 

 imminent. To every one is known the enormous expansive power of 

 growing roots, and it will readily be seen how very severely a large 

 mass of them may affect a rock that has, for instance, the physical 

 constitution of a sandstone. 



Flowing water, with or without sand and detritus in suspension, is 

 one of the most directly acting agents, and is productive of results upon 

 a grand scale. Analogous thereto, though more restricted in occurrence, 

 is the action of moving ice. Precipitated water i^resents results similar 



