236 LECTURE IX. 



extract from the rocks certain elements, and that this extrac- 

 tion is nowhere greater than in the gypseous and chalk 

 mountains ; because in these latter the dissolving power of 

 simple water is augmented by the addition of carbonic acid, 

 which is found in all atmospheric waters. For this reason, a 

 deposit will be specially formed in such fissures, where a 

 small quantity of water trickles down slowly and partly evapo- 

 rates, whilst, on the contrary, where large quantities of water 

 pass through rapidly, the fissure becomes rather enlarged by 

 the removal of its contents. The origin of the large cavities 

 in horizontal fissures is, however, mainly attributable to the 

 falling in of beds deprived of their supports, which thus form 

 wide spaces in the interior of the mountains. 



All these phenomena, from the finest crack to the largest 

 cavern, are closely connected ; their formation being confined 

 neither to time nor jjlace ; the filling up depending on local 

 conditions. Where there is no access for the water from above, 

 it can only enter from the sides or springs which may rise up 

 from beneath. If there be external apertures, springs, brooks, 

 and streams may enter and form a subterraneous river system, 

 • as actually existing in many spots, but nowhere so well deve- 

 loped as in Carinthia and the Krain on the platform above 

 Trieste, where is found a series of subterranean lakes connected 

 by rivers, partly navigable, and inhabited by various animals. 



It is customary to make a distinction between crevices of 

 Httle width, running more or less perpendicularly ; grottoes, 

 which are but short cavities, with externally wide apertures ; 

 and caverns, consisting of a succession of vaults, connected by 

 narrow channels. The grottoes, or balmen as they are called, 

 (probably from a Celtic word) in Switzerland, South Germany, 

 and France, frequently owe their origin to soft beds of marl, 

 carried ofi" from beneath the harder lime beds, which now cover 

 them ; they are often only the inlet of caverns cut ofi" by the 

 closure of the fissure. Caverns, on the contrary, are sometimes 

 of surprising dimensions, extending in some cases for miles, 

 under the ground, frequently containing chambers one hundred 

 feet in height, and as many in diameter. These chambers are 

 not always situated in the same plane, but have to be reached 



