CHEMICAL ORIGIN OF CAVERNS. 131 



gas itself in all cases an undoubted evidence of 

 chemical decompositions at a depth below the 

 surface is the cause of important chemical 

 changes in the crust of the earth. Feeble as the 

 agency may appear, the gas, being a weak acid, 

 exerts in reality a most powerfully decomposing 

 and disintegrating effect upon the different strata 

 through which it is compelled to permeate in 

 its passage to the surface. Strata, which would 

 otherwise remain solid and intact for centuries, 

 are ready to crumble to pieces in consequence. 

 Dissolving out, by its solution in water, many 

 of the elements of the rocks, it eventually occa- 

 sions them to become quite soft, or even (as in 

 the case of limestone rocks) it may produce those 

 great caverns and long galleries, of which dif- 

 ferent countries present us with such curious and 

 magnificent specimens.* When the solution 

 thus obtained reaches the surface, it there loses 

 its carbonic acid, and deposits its calcareous 

 matter (carbonate of lime) in the form of a white, 

 solid, stony mass, called " travertin." A remark- 

 able example of water fully charged with carbonic 



* At Fredericshall, in Norway, is a cavern 11,000 feet deep. 

 The most celebrated cave is at the village of Adelsberg, hi 

 Austria. The mere vestibule, called the Dome, to this mag- 

 nificent cavern, is upwards of 100 feet high, and more than 

 300 feet deep. The entire extent of the cavern cannot be 

 ascertained ; it is known to be very great. Yet all appears 

 to be due to the action of water and carbonic acid ! 



