706 STUDIES FOR STUDENTS 



insignificant amount of the decomposition to the action of car- 

 bonic acid produced through the instrumentality of ants. 



The chemical changes involved in the process of decompo- 

 sition received attention from several of the earlier workers, 

 among whom the names of Berthier, J. G. Forschhammer, 

 Brongniart, Gustav Bischof and Ebelmen stand out in greater 

 prominence. More recently the name of Sterry Hunt becomes 

 conspicuous, while the purely geological side of the question has 

 been set forth in numerous papers by De La Beche, L. Agassiz, 

 R. Pumpelly, N. S. Shaler, O. A. Derby, J. C. Branner and others, 

 to whom reference is made in these pages. 



(2) THE AGENCIES ENGAGED IN PROMOTING ROCKWEATHERING. 



The expression " rockweathering " as commonly used, is a 

 comprehensive term descriptive of the processes which are so 

 constantly engaged in degrading rock-masses and reducing them 

 to the condition of gravel, sand, and clay, and incidentally to 

 soil. These processes are in part physical and in part chemical 

 in their nature ; at times simple, and yet again complex. But, 

 whatever the forces engaged, they are, with a few isolated excep- 

 tions, superficial — they work from the surface downwards. 

 However much they may have accomplished since the first rock- 

 masses appeared above the primeval ocean, in no case can the 

 actual amount of debris in situ, have formed at one time more 

 than a scarcely appreciable film over the underlying and 

 unchanged material. The decomposing forces early lose their 

 active principles, and become quite inert at depths comparatively 

 insignificant. It is only where through erosion the results of 

 the disintegration are gradually removed, that the processes 

 have gone on to such an extent as to perhaps quite obliterate 

 thousands of feet of massive rock, and furnished the necessary 

 debris for the great thicknesses of sandstone, slate, and shale, 

 which characterize the more modern horizons. In certain iso- 

 lated cases, it is true, ascending steam and heated waters arising 

 from unknown depths, have been instrumental in promoting 

 decomposition, as is well illustrated in the areas of decomposed 



