AGENCIES PROMOTING DISINTEGRATION. 381 



of much more advanced decomposition, being in most cases at the im- 

 mediate surfoce reduced to the condition of residual clays. 



Causes of Disintegration. 



It is evident from what has gone before that the changes which have 

 taken place in tlie mass of rock are as much in the nature of disintegra- 

 tion as decom})osition. The question, then, very ])r()mptly arises, what 

 are the agencies whicli have been instrumental in bringing about a disin- 

 tegration which in extreme cases extends to a depth of 50 feet and 

 upward. 



It is customary to divide the forces commonly active in promoting 

 rock-weathering into two groups — physical and chemical. Of the phys- 

 ical agencies, temperature changes alone need be considered in this con- 

 nection ;* of the chemical agencies, oxidation, hydration and solution. 



It has been abundantly demonstrated in the work of the various ex- 

 periment stations that at a depth of a few inches beneath the surface the 

 daily variation in temperature is very slight, and we may safely assume 

 that at depths of a few feet both the annual and daily variations are also 

 so small as to be practically inoperative. The purely physical agencies 

 may be therefore omitted from further consideration. 



Of the chemical agencies, it is evident that the process of solution has 

 not been sufficiently active to carry away more than an extremely small 

 proportion of the material, but has contented itself with bringing a frac- 

 tional part of the elements into a new state of combination. These facts, 

 would seem to render it very doubtful if bacterial agencies, as suggested 

 by A. Miintz and others,t have operated to any appreciable extent. 



Oxidation has manifested itself in the superficial portions in the par- 

 tial destruction of the protoxide silicates, but even this action to a large 

 extent ceases at a depth of 20 feet below the surface. 



Of all the agencies enumerated, hydration seems most pronounced and 

 most nearly universal. Now, hydration in a rock-mass, without loss of 

 any constituent, necessitates expansion, and as the various minerals 

 undergoing this process will expand unequally, a tendency toward dis- 

 integration is manifested, even when the ])r()cess has stop})ed short of the 

 complete kaolinization of the feldspars. This fact was impressed upon 

 me in a very striking manner some years ago, and inasmuch as I do not 



• In tlie rJiHciiMHion which followed the rending of this paper before the GeoloKiciil Society of 

 Wa.HhinRton in Jiinuary, lH9o, thf question wns niised as to the possible efficiicy of capillurity in 

 promotin« disiiiteKration. The writer ean only say that he is unable to conceive of the direct 

 physical action of oapillarity as being other than neutral. As a secondary factor in promoting 

 hydration, it is undoubtedly of importance. 



tComptes ReoduB de TAcademie des Sciences, vol. ex, 1890, p. 1.370. 



