CAVES AND CAVE DEPOSITS. 165 
that commonly fills such cracks is easily removed, and the 
various denuding agencies are apt to follow fissures, and thus 
caves be formed along them. That is why it so often happens 
that mines break into caves, or trial holes are made in caves, the 
fissure vein or lode being seen in the cavernous rock. 
The unequal flow of lava curling and coiling over the half- 
cooled mass of earlier flows sometimes leaves openings like 
caves; and it is said that some of the caves in volcanic districts 
are opened out by the various acidic vapours which act on the 
micaceous and other schistose rocks which have been already 
fissured by the earthquakes so frequent in those countries—as, 
for instance, in the case of some of the caves of Corinth and 
the Cyclades. 
These are, however, few and unimportant, seldom occurring 
where a cave would be much frequented by man or the lower 
animals. 
The commonest caves, and those which generally have proved 
of greatest interest, are the old subterranean watercourses so 
frequent in limestone rocks. The way in which these caves are 
formed is well known, but many of the phenomena connected 
with them appear to be less clearly understood, and so we hear 
of various startling theories propounded which, on enquiry, turn 
out to be based on a wrong interpretation of the mode of 
formation of the deposits found in such caves. It is to these 
questions I especially invite attention tonight, and in the selec- 
tion of examples in illustration I desire to explain the mode 
of formation of the cave earth and laminated clays, stalagmitic 
floors, and broken-up travertine breccias, stream-gravel, and 
angular talus, and to make clear the distinction between the 
age of the caves and of the cave deposits. 
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 implies the never-ceasing operation 
of the agencies which tend to form caves. 
