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 porphyry produce 
a red soil, and graywacke a light gray soil, full of 
smooth, rounded pebbles, evidently derived from 
the imdecomposed 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 calc 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 
