DEGRADATION. 183 



fied ; the latter yield more readily than the former. The strata may be thick 

 or thin; the latter yield more readily than the former. The strata may be 

 horizontal or inclined, and the latter yield more readily than the former; 

 and in each condition above mentioned heterogeneity promotes disinte- 

 gration. 



2. Litholoffic Structure. Under this head we may consider whether the 

 beds are compacted crystals or cemented sediments; the latter yield more 

 readily than the former, and heterogeneity promotes disintegration. Both 

 in detrital and crystalline rocks there are certain conditions which may be 

 grouped under the head of induration, including hardness and toughness, and 

 the greater the induration the greater the stability; and the law of hetero- 

 geneity applies also to this condition. 



3. Chemical Structure. The principal condition of chemical constitu- 

 tion relating to endurance is solubility; the soluble yield more readily than 

 the insoluble, and in chemical constitution the homogeneous is more stable 

 than the heterogeneous. 



It will thus be seen that in each particular mentioned above, the homo- 

 geneous is more stable than the heterogeneous, and these particulars in their 

 correlations are themselves conditions of heterogeneity, so that the whole 

 subject of petrology in its relation to disintegration, is one of degree of 



heterogeneity. 



DYNAMICS OF DISINTEGRATION. 



The principal forces of disintegration are gravity, heat, crystallization 

 and chemical reaction. Gravity disintegrates the rocks directly where they 

 break from cliffs and ledges partly by their own weight, and are further dis- 

 integrated by the fall, and indirectly through the agency of water in abra- 

 sion. Heat disintegrates the rocks by change of temperature, and probably 

 by expansion of water permeating the rocks. Crystallogenic force also acts 

 through the agency of water, for where the water which has permeated the 

 rocks is frozen, the expansion due to this crystallization breaks them asun- 

 der. In chemical reaction the rocks are broken up through the agency of 

 water which acts directly in dissolving them, or indirectly in promoting 

 other chemical reactions. 



