ON CAVES. 83 
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 
ina 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. Hven 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 analyses 
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 débris 
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 
