ON CAVES. 15 



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 primaeval 

 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, and is probably the chief agent 

 in case of a rock undercut so far that the rain cannot touch it, 

 just as Renduf 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 where does it go to, 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, Reliquiae Aquitanicce, 1876. 

 t Eendu, Theorie det Glaciers. 



