DISINTEGRATION OF ROCKS. 149 



In a red sandstone district we find the soil of a red 

 colour, and made of grains of quartz, with oxide of 

 iron and clay, and a few particles of undecomposed 

 mica. So also red slate and red forfhyry produce 

 a red soil, and graywacke light gray soil, full of 

 smooth, rounded pebbles, evidently derived from 

 the undecomposed fragments of the rock. 



Some of the above rocks decompose more readi- 

 ly than others ; but the correspondence of the soil 

 with the characters of the underlying rocks proves 

 very conclusively that it has been derived from the 

 decomposition of the latter. 



M. Daubuisson, an eminent French geologist, 

 states that in a roadway in France, cut through 

 granite, the rock was decomposed to the depth of 

 three inches in less than six years. 



" The disintegration of granite," says Lyell, " is 

 a striking feature of large districts in Auvergne. 

 This decay was called by Dolomieu 'la maladie du 

 granite,' and the rock may, with propriety, be said 

 to have the rot, for it crumbles to pieces in the 

 hand. The phenomenon may, without doubt, be 

 ascribed to the continual disengagement of carbonic 

 acid gas from numerous fissures. In the Plains of 

 the Po I observed great beds of alluvium, consist- 

 ing of primary pebbles, percolated by spring water, 

 charged with carbonate of lime and carbonic acid 

 in great abundance. They are, for the most part, 

 incrusted with caic sinter; and the rounded blocks 

 of gneiss, which have all the outward appearance of 

 solidity, have been so disintegrated by the carbonic 

 acid as readily to fall to pieces. 



'' The subtraction of many of the elements of rocks 

 by the solvent power of carbonic acid, ascending 

 both in a gaseous state and mixed with spring- 

 water in the crevices of rocks, must be one of the 

 most powerful sources of those internal changes 

 and rearrangement of particles so often observed 

 in strata of every age- The calcareous matter, for 



