ATMOSPHERIC EFFECTS. 1.23- 



certain granites, and in dressed stone. The effect is more rapid 

 and perceptible, in proportion to the susceptibility of the substance 

 to imbibe moisture, and to dry again; alternations which produce 

 a very rapid disaggregation, when frequently repeated, as is gene- 

 rally the case in mountains. The substances which degrade most 

 easily, are those of a granuhr structure, either earthy or crystalline ; 

 those of a foliated structure ; or compact masses, fractured and 

 split on the surface, such as are often seen in mountains. Frost, 

 when it attacks water absorbed by a body, is also a powerful cause 

 of destruction, because the expansion consequent upon it produces 

 a multitude of cracks in all directions. As long as the cold con- 

 tinues, its parts are held together by ice as by a cement ; but when 

 a thaw comes, the whole falls in scales, grains, or dust. 



Mountains cannot be visited without meeting evident traces of degrada- 

 tion of this kind. In limestone escarpments (Jig. 204), we see parts of loose 



Fig. 204. Fig. 205. 



Daily effects of degradation in mountains. 



texture, more or less hollowed out, and the more solid banks remain. Hence 

 the falling of the latter, which are successively detached in more or less 

 voluminous blocks. In high mountains (Jig. 205), often formed of in. 

 clined strata, which present their cuts or planes to the slope, we observe the 

 most marked degradations : parts are constantly detached, particularly at 

 times of most sensible atmospheric variations; at the instant of thaw, enor- 

 mous avalanches of stones occur, and roll down the sides with astonishing 

 rapidity, sweeping everything in their course ; sometimes great blocks, and 

 considerable portions of the mountain fall with tremendous noise. Hence 

 the enormous debris which accumulate at the base, sometimes covering a 

 great extent. 



& Degradations attributable to these effects. The degradation 

 which many rocks present is generally attributed to atmospheric 

 influences, long continued. Almost all rocks, in fact, are more or 

 less deeply changed, and are in a state of much less solid aggrega- 

 tion, much less homogeneous, on the surface, than they are inter- 

 nally. In almost all quarries, it is necessary to remove a great mass 

 of matter, before obtaining blocks which are homogeneous, solid, 

 free from cracks, and possessed of the bright colours which art 

 ordinarily sought; this is especially the case with marble, and 

 generally, also, with compact limestone. Certain granites are so 

 deeply disintegrated, that the whole surface of the soil presents a 



2. What is meant by degradation of rocks? What are rocking gtonea ,' 



