DISINTEGRATION OF ROCKS. 115 



alternations of moisture and dryness which must occur 

 where, during a great part of the year, the hills are 

 covered with fog in the morning and exposed to the 

 sun in the afternoon. 



In connection with this subject I may remark that, 

 in countries where the rainfall is very slight or alto- 

 gether deficient, we are apt to be misled by the appear- 

 ance of the surface, and to much overrate the real 

 amount of disintegration. In the drier parts of the 

 Mediterranean region, especially in Egypt, as well as in 

 Peru and Chili, we constantly see rocky slopes covered 

 with fine debris which represent the accumulated work 

 of many centuries, remaining in situ because there 

 is no agency at work to remove it, while in countries 

 where the slopes are frequently exposed to the action 

 of running water fresh surfaces are subjected to the 

 action of the atmosphere, and the comminuted materials 

 are carried to a distance to form alluvial fiats, to fill 

 up lakes, or ultimately to reach the sea-coast. A 

 somewhat similar remark may be made with regard 

 to rock surfaces habitually covered with snow and 

 very rarely exposed to heavy rain. I have often ob- 

 served in the Alps and Pyrenees that, when the snow 

 disappears during the short summer of the higher 

 regions, we generally find the surface covered with 

 small fragments of the underlying rock, not removed 

 by the slow percolation of water during the melting 

 of the snow. The same phenomenon long ago at- 

 tracted the attention of Darwin during his short 

 excursion across the passes of the Chilian Andes. 



I regretted much that my very short stay at Lima 

 left me no time to visit the places where these curious 



