274 SCIENCE PROGRESS 



they got there. I remember a hypothesis being advocated 

 that man arrived here in a cold period, when the rivers were 

 frozen over, and that he made air-holes in the ice for the fish 

 to come to, when they were speared, and through which the 

 implements got lost in the rivers and mixed with the gravels. 

 Even now there are many people who have not fully realised 

 the chronological value of implements in a deposit. That 

 they are fossils no one can deny, and they must be treated as 

 such ; but they differ greatly as chronometers when com- 

 pared with the mollusca that may lie beside them. When 

 one sees the delicate little double valves of the mollusca still 

 unseparated, and a faunal assemblage which have a common 

 habitat, we may be sure they are in their original home, and 

 were contemporaneous with the deposit. As we examine the 

 fauna of a particular river-valley at the different altitudes 

 we are able to trace the history of the latter : whatever its 

 depth may be we can see that the valley has been excavated 

 in (say) pleistocene times ; widely separated as its banks now 

 are, time was when the valley was filled by the country rocks 

 which at one time formed the surface of the earth, upon which 

 man and beast lived ; from the commencement of that excava- 

 tion the stream has been carrying seaward the soluble and 

 transportable materials, ever encroaching upon plateau and 

 plain, cutting back subsequent and obsequent " land-grabbers," 

 and constantly drawing into its vortex rocks, stones, and 

 earth. But although its operations may have been extensive 

 they were nevertheless limited, and vast areas of surface have 

 never been denuded of their surface materials. Naturally 

 during the whole of this time the primaeval hunters have 

 camped beside its waters, and sat upon its banks and worked 

 flints into implements which in many cases have been pre- 

 served practically as man left them. In other cases they 

 have been whirled along in the waters and mixed with others 

 belonging to long anterior ages. Sometimes an easier-wearing 

 stratum has been encountered at some distance, and the stream 

 has gone into the deeper channel and left the deposit (and 

 perhaps the flaking floor) as a terrace, on the sides of the 

 valle}^. The same thing, of course, occurs with the alteration 

 of land altitudes, and base levels of erosion. I submit there 

 are two very obvious facts that are not usually recognised. 

 Firstly, that the area of old land surfaces that have not 



