ON CAVES. 7 



in such caves. It is to these questions I especially invite 

 attention to-night, and in the selection of examples in illus- 

 tration I shall be chiefly guided by the desire to make clear 

 the distinction between the age of the caves and of the cave 

 deposits, and the mode of formation of the cave earth and 

 laminated clays, stalagmitic floors, and broken-up travertine 

 breccias, stream-gravel, and angular talus. 



First, I would just remind you that these caves are formed 

 in a rock which can not only be mechanically broken up and 

 carried off, but can also be dissolved in water and carried 

 away in solution wherever water can pass. Even pure water 

 can take up two grains per gallon of carbonate of lime, of 

 which these rocks are largely composed. But pure water is very 

 rarely found in nature. The rain generally takes up some 

 carbonic acid from the air, and when it falls on the ground 

 gets a great deal more from the decomposing vegetation, and 

 water with carbonic acid in it acts rapidly upon the limestone 

 rock, carrying off part of it as a bicarbonate of lime, while the 

 earthy part is washed away in mechanical suspension till it 

 settles down in some pool of still water as mud, often forming 

 a considerable part of the cave-earth which fills all the 

 interstices of the broken rock. As may be seen by the analysis 

 of hard waters, it is not uncommon to get 25 grains per 

 gallon of carbonate of lime in the water of limestone districts, 

 and this means the never-ceasing operation of the agencies 

 which tend to form caves. 



So, of course, the most favourable conditions for the for- 

 mation of such caves are First, a limestone into which the 

 water can trickle down along joints and fissures, and find its 

 way out at some lower level. Secondly, an area over which the 

 rain can gather into streamlets and collect from vegetation 

 the acids which will help it to dissolve the rock. The crack 

 into which the water first finds its way may be very small ; the 

 water soon opens it out, acting first chemically, then mechani- 

 cally, on the surrounding rock. When the sand and broken 

 rock get a free passage, mountain torrents, full of debris 

 torn from the hill-side or washed out of ancient boulder- clays, 

 are precipitated into the chasms, which take the place of the 

 half-opened joint, and the work goes on apace. 



It is quite clear that in such circumstances it must often 

 happen that, as the clay or shale on the hill-side is being 

 denuded away, the water must find its way into the jointed 

 limestone further and further back continually, and, in the 

 deep recesses of the mountain, new channels must often carry 

 off the water that once ran higher up. Thus, the higher out- 

 falls are left dry, and then they are in a state for man and 



