James Geikie — On Changes of Climate. 221 



interglacial deposits must have been ploughed out again and again 

 whenever a glacial period supervened, and even during interglacial 

 periods subaerial denudation would carry on the work of destruc- 

 tion ; in short, the deposits of each period would be made up 

 to a large extent of pre-existing drifts. But as we recede from the 

 uplands and approach the low country, we get upon ground where 

 glacial action would be less intense, and where, consequently, inter- 

 glacial deposits would stand a better chance of preservation. 



But it may still be objected that if the older river-gravels are 

 really of interglacial age, we ought to find them in some places at 

 least covered by glacial deposits. Now I will not stop to ask how 

 often this may have been the case without the glacial character of 

 the overlying deposits having been recognized. Hitherto, the post- 

 glacial age of the valley-gravels has been by many assumed, not proved. 

 Yet it is not too much to say that the mere absence of an overlying 

 cap of glacial clay or sand, or silt or gravel, is not sufficient evidence 

 of the postglacial age of a deposit. Such an overlying bed may 

 have been removed by denudation, or it may never have existed. But 

 we do find in the valley -gravels themselves evidence of cold climatal 

 conditions. I need only refer to the well-known occurrence of the 

 travelled stones and boulders which Mr. Prestwich thinks may point 

 to transport by means of floating-ice ; and the confused and tumul- 

 tuous appearance which many of the older river-deposits present may 

 also, he believes, be accounted for by the grounding of heavy ice- 

 rafts. This explanation at once commends itself by its simplicity ; 

 but whether the transport of the boulders and the confused bedding 

 of the gravels have in all cases been effected by river-ice is at least 

 doubtful. Granting, however, that they have been, and that the 

 deposits in which these phenomena occur are of postglacial age, it is 

 quite clear that the conditions of climate which Mr. Prestwich's 

 theory demands could not have been suited to the wants of the 

 hippopotamus. Nor could we reasonably expect to find a mammalian 

 fauna at once so varied and abundant as that of our river-gravels 

 and cave-deposits, living in England at a time when the winters 

 were so intensely severe. • Is it not more reasonable, then, to con- 

 clude, that the southern forms had already left Britain before the 

 climate had so cooled down as to permit the presence of large ice- 

 rafts on our rivers ; in other words, that the travelled boulders and 

 confused bedding point either to the approach or the disappearance 

 of a cold or glacial period ? 



But still stronger evidence in this direction is furnished us by the 

 "Trail" so lucidly described by the Eev. 0. Fisher. If this peculiar 

 deposit be not due to the action of land-ice, as Mr. Fisher contends, 

 to what other cause can we ascribe it ? The chief objection which 

 it appears to me can be urged against the explanation given is only 

 our own preconceived idea that valley-gravels as a rule must belong 

 to a later date than the Glacial epoch. But freshwater deposits 

 of interglacial age occur in Scotland, Switzerland, and America,' and 



' In a former paper (Geol. Mag., Vol IX., p. 64) I referred to the " Hardpan" 

 of North America as being in all probability the equivalent of our Till. Since then 



