THE. QUATERNARY PERIOD. 341 
excavation of the valley had proceeded to a much less extent. 
These (fig. 255, 4 4’) are the so-called “ Aigh-level gravels” 
of a river, and there may be one or more terraces of these. 
The important fact to remember about these fluviatile de- 
posits is this—that here the ordinary geological rule is reversed. 
The high-level gravels are, of course, the highest, so far as 
their actual elevation above the sea is concerned ; but geo- 
logically the lowest, since they are obviously much older than 
the low-level gravels, as these are than the recent gravels. 
How much older the high-level gravels may be than the low- 
level ones, it 1s impossible to say. They occur at heights 
varying from 10 to 100 feet above the present river-chan- 
nels, and they are therefore older than the recent gravels 
by the time required by the river to dig out its own bed to 
this depth. How long this period may be, our data do not 
enable us to determine accurately ; but if we are to calculate 
from the observed rate of erosion of the actually existing 
rivers, the period between the different valley-gravels must 
be a very long one. 
The lowest or recent fluviatile deposits which occur beside 
the bed of the present river, are referable to the Recent period, 
as they contain the remains of none but living Mammals. The 
two other sets of gravels are Post-Pliocene, as they contain 
the bones of extinct Mammals, mixed with land and fresh- 
water shells of existing species. Among the more important 
extinct Mammals of the low-level and high-level valley-gravels 
may be mentioned the Eilefhas antiquus, the Mammoth (£é- 
phas primigenius), the Woolly Rhinoceros (2. tichorhinus), the 
Hippopotamus, the Cave-lion, and the Cave-bear. Along 
with these are found unquestionable traces of the existence 
of Man, in the form of rude flint implements of undoubted 
human workmanship. 
The so-called ‘“ Cave - deposits,” again, though exhibiting 
peculiarities due to the fact of their occurrence in caverns or 
fissures in the rocks, are in many respects essentially similar 
to the older valley-gravels. Caves, in the great majority of 
instances, occur in limestone. When this is not the case, it 
will generally be found that they occur along lines of sea-coast, 
or along lines which can be shown to have anciently formed 
the coast-line. ‘There are many caves, however, in the making 
of which it can be shown that the sea has had no hand; and 
these are most of the caves of limestone districts. These owe 
their origin to the solvent action upon lime of water holding 
carbonic acid in solution. The rain which falls upon a lime- 
stone district absorbs a certain amount of carbonic acid from 
