156 
ELEMENTS OF GEOLOGY. 
river had thrown down and afterwards removed alluvial 
matter. 
Cavern Deposits containing Human Remains and Bones of 
Extinct Animals. —In England, and in almost all countries 
wdiere limestone rocks abound, caverns are found, usually 
consisting of cavities of large dimensions, connected together 
by low, narrow, and sometimes tortuous galleries or tunnels. 
These subterranean vaults are usually filled in part with 
mud, pebbles, and breccia, in which bones occur belonging 
to the same assemblage of animals as those characterizing 
the Post-pliocene alluvia above described. Some of these 
bones are referable to extinct and others to living species, 
and they are occasionally intermingled, as in the valley- 
gravels, with implements of one or other of the great divi¬ 
sions of the stone age, and these are not unfrequently accom¬ 
panied by human bones, which are much more common in 
cavern deposits than in valley-alluvium. 
Each suite of caverns, and the passages by which they com¬ 
municate the one with the other, afford memorials to the ge¬ 
ologist of successive phases through which they must have 
passed. First, there was a period when the carbonate of 
lime was carried out gradually by springs; secondly, an era 
when engulfed rivers or occasional floods swept organic and 
inorganic debris into the subterranean hollows previously 
formed; and thirdly,there were such changes in the configu¬ 
ration of the region as caused the engulfed rivers to be turn¬ 
ed into new channels, and springs to be dried up, after which 
the cave-mud, breccia, gravel, and fossil bones would bear the 
same kind of relation to the existing drainage of the country 
as the older valley-drifts with their extinct mammalian re¬ 
mains and works of art bear to the present rivers and allu¬ 
vial plains. 
The quarrying away of large masses of Carboniferous and 
Devonian limestone, near Liege, in Belgium, has afforded the 
geologist magnificent sections of some of these caverns, and 
the former communication of cavities in the interior of the 
rocks with the old surface of the country by means of ver¬ 
tical or oblique fissures, has been demonstrated in places 
where it would not otherwise have been suspected, so com¬ 
pletely have the upper extremities of these fissures been 
concealed by superficial drift, while their lower ends, which 
extended into the roofs of the caves, are masked by stalac- 
titic incrustations. 
The origin of the stalactite is thus explained by the emi¬ 
nent chemist Liebig. Mould or humus, being acted on by 
moisture and air, evolves carbonic acid, which is dissolved 
