70 Rainfall, Natural Drainage, 



of some feet or even yards. Wherever this discoloration 'has 

 taken place, the rock is softer and contains more moisture, and 

 the surface is full of crevices : here the work of destruction has 

 begun. In the Channel Islands, remarkable for their excellent 

 granite, there may be seen at least twenty feet of the stone on 

 which part of the town of St, Peter Port, Guernsey, is built 

 reduced to such a state that it can be dug out with a spade. In 

 Jersey, behind St. Aubin's, there is a thickness of at least thirty 

 feet of a kind of gravel, which is nothing more than the loosened 

 crystals of the felspar and quartz of the granite decomposed by 

 weathering, and quite disintegrated. Close by, the same rock 

 juts out in isolated masses into the sea. In Alderney the centre 

 of the island contains innumerable round boulders of granite 

 entirely the result of decomposition ; and yet from these islands 

 are obtained some of the hardest, toughest, and most enduring 

 granites in common use. So again I remember seeing among 

 the grand basaltic columns of the Giant's Causeway, in the 

 North of Ireland, the roots of plants twined round the slices of six- 

 sided columns ten or fifteen feet below any point exposed to the 

 air. The rock itself seems indestructible, but the traces of decay 

 are evident on close investigation. These are but a few instances 

 out of many in which I have seen crystalline rock affected by 

 weather to a great depth without any other cause than the pene- 

 tration of moisture by absorption. 1 may safely say that I have 

 never in any part of the world seen a natural or artificial face of 

 granite or basalt exposed without finding evidence of the destroy- 

 ing power of weather, acting always by aid of water penetrating 

 within the mass. Besides destruction of this kind, the joints of 

 granite frequently contain water, and sometimes yield it in large 

 quantity from artesian wells. 



Sandstones of all kinds exhibit weathering, and water pene- 

 trates them to great depths. The softer varieties of sandstone 

 are easily cracked during even a short continuance of dry weather. 

 When rain comes it fills these fissures, and penetrates yet more 

 deeply. All sandstones are more or less porous, and in this way 

 admit moisture. All are more or less distinctly bedded, and 

 they generally allow water to pass along in the intervals between 

 the beds. All, again, are more or less cracked and fissured at 

 the surface. The chemical effect of water in dissolving and 

 decomposing is less seen in sandstones than in granites, except 

 when the sandstone is impure, and contains marl, calcareous 

 cement, or a mixture of mica and felspar. In such cases the 

 result is soon seen, and is often very great. On the whole, there 

 are no rocks that admit water so slowly as pure crystalline sand- 

 stones and quartzlte, and none that are more absorbent than soft, 

 loose, rotten sands Avith which marl is intermixed. 



