ON CAVES. 91 
would wash them into caves and form an ossiferous deposit 
like that in the caves of Franconia. 
There can be, however, no general explanation for all bone- 
bearing caves. We must examine all the evidence in each 
case, and then form our opinion as to how a particular bone- 
bed was formed. Buckland’s view seems to me to be in most 
cases the correct one. 
So are caves formed and modified, and filled and swept 
clean and filled again, and we must bear all these facts in mind 
when we attempt to read the story of a cave from the deposits 
which we find in it. 
Broken-up stalagmitic floors are not evidence of the action 
of the sea, but, on the contrary, must generally be referred to 
land floods, 
Laminated clays are not evidence of glacial action, but only 
of alternations of muddy and clear water, such as follow rainy 
and fair weather. 
Some of the most interesting caves, in respect of their con- 
tents and the light they throw on the history of primeval 
man, are only rock-shelters—abris—such as are seen in 
the Dordogne district.* They are sometimes longitudinal 
sections of parts of subterranean watercourses, but are more 
commonly due to the weathering away of soft rock between 
two harder beds. It does not always require a stream or direct 
rainfall to wet the surface of a rock sufficiently to let the frost 
act upon it. The travelling moisture of the air, condensed in 
and on the cold rock, is enough, andis probably the chief agent 
in case of a rock undercut so far that the rain cannot touch it, 
just as Rendut explains the film of ice upon the snow at high’ 
elevation not by the melting and refreezing of the snow, but 
by the condensation of the little moisture left in the air which 
comes in contact with the snow in those high regions. 
The carbonate of lime of the limestone is removed by the 
water and carbonic acid ; but whither does it go, and what 
becomes of the earthy residuum which forms so large a part 
of some limestones? ‘These can also be traced, and furnish 
us with evidence of another kind that this subterranean 
chemical denudation is going on. When the acidulated 
water falls upon chalk, for instance, and, instead of being 
collected into rivulets, acts over the whole surface, we 
find a great mass of red clay, full of flints which have been 
weathered out. A great part of this red clay is the insoluble 
portion of the chalk. All limestones have a good deal of iron 
* Lartet, Christy, and Jones, Reliquiw Aquitanice, 1876. 
+ Rendu, Theorie des Glaciers. 
