88 SCIENCE PROGRESS 



ferent explanations are offered. One school points to the 

 gravels of the Kentish plateau, in which these implements 

 sometimes occur, as proved by their position and composition 

 (containing as they do Greensand chert from the Wealden area) 

 to belong to the very earliest stages in the excavation of the 

 Thames valley ; so that if the implements are really, as 

 alleged, contemporary with the gravel, then they must be far 

 older than the palaeoliths of the ioo ft. terrace ; and in proof 

 of this they adduce a certain number of these " hill-types," 

 recognisable by their colour, which have been found in the 

 gravels of that terrace in such a condition as to prove that 

 they are derived. If this view is accepted, it seems that both 

 Chellean and Acheulean peoples have visited this country twice 

 at widely separated periods, presumably in connection with 

 cycles of climatic change. 



Other authorities, however, while admitting the general 

 antiquity of the high-level gravels, believe that they have been 

 subject to much disturbance at later dates, quite apart from 

 river action, and may in some cases have moved in large 

 masses down the hillsides in a semi-frozen condition, burying 

 implements which originally lay on the surface. The presence 

 of hill-types on the ioo ft. terrace is also admitted, but since 

 the highest gravel (upper gravel) there is shown by Messrs. 

 Smith and Dewey to be a hill-wash of later date than the 

 Acheulean implements, the occurrence in it of some of these 

 specimens, washed from the plateau above, is by no means 

 unintelligible. The same arguments apply to the Hackpen 

 Hill implements, while the Limpsfield site is obviously one 

 which might receive hill-wash of much later date than the old 

 river-plain on which the gravels rest. 



Near Farnham two waterworn Chellean implements are 

 alleged to have been found at 600 ft. O.D., and as there is no 

 higher ground from which they can have been washed, it 

 seems hardly possible to account for their abraded condition 

 by mere disturbance of an old gravel by frost or otherwise. 

 If therefore they could be proved to have come from this level, 

 it would be necessary to attribute to them an age far greater 

 than that of the 100 ft. terrace ; unfortunately, however, we 

 have nothing but the testimony of the workmen, which can 

 hardly be considered sufficient. 



On the whole the evidence at present available seems in 



