342 HISTORICAL PALAZZONTOLOGY. 
the air, or from the soil. It then percolates through the rock, 
generally along the lines of jointing so characteristic of lime- 
stones, and in its progress it dissolves and carries off a certain 
quantity of carbonate of lime. In this way, the natural joints 
and fissures in the rock are widened, as can be seen at the 
present day in any or all limestone districts. By a continu- 
ance of this action for a sufficient length of time, caves may 
ultimately be produced. Nothing, also, is commoner in a 
limestone district than for the natural drainage to take the 
line of some fissure, dissolving the rock in its course. In this 
way we constantly meet in limestone districts with springs 
issuing from the limestone rock—sometimes as large rivers— 
the waters of which are charged with carbonate of lime, ob- 
tained by the solution of the sides of the fissure through which 
the waters have flowed. By these and similar actions, every 
district in which limestones are extensively developed will be 
found to exhibit a number of natural caves, rents, or fissures. 
The first element, therefore, in the production of cave-deposits, 
is the existence of a period in which limestone rocks were 
largely dissolved, and caves were formed in consequence of 
the then existing drainage taking the line of some fissure. 
Secondly, there must have been a period in which various 
deposits were accumulated in the caves thus formed. These 
cavern-deposits are of very various nature, consisting of mud, 
loam, gravel, or breccias of different kinds. In all cases, these 
materials have been introduced into the cave at some period 
subsequent to, or contemporaneous with, the formation of the 
cave. Sometimes the cave communicates with the surface by 
a fissure through which sand, gravel, &c., may be washed by 
rains or by floods from some neighbouring river. Sometimes 
the cave has been the bed of an ancient stream, and the de- 
posits have been formed as are fluviatile deposits at the surface. 
Or, again, the river has formerly flowed at a greater elevation 
than it does at present, and the cave has been filled with 
fluviatile deposits by the river at a time prior to the excava- 
tion of its bed to the present depth (fig. 256). In this last 
case, the cave-deposits obviously bear exactly the same rela- 
tion in point of antiquity to recent deposits, as do the low- 
level and high-level valley-gravels to recent river-gravels. In 
any case, it is necessary for the physical geography of the dis- 
trict to change to some extent, in order that the cave-deposits 
should be preserved. If the materials have been introduced 
by a fissure, the cave will probably become ultimately filled 
to the roof, and the aperture of admission thus blocked up. 
If ariver has flowed through the cave, the surface configura- 
