On British Drifts and the Interglacial Problem. 305 



both in this country and abroad, is with regard to the value of 

 the evidence for interglacial periods ; and it will be my aim, in 

 bringing- before you some general conclusions regarding the 

 drifts, to concentrate attention principally upon this evidence. 



To keep the discussion within practicable limits I must 

 perforce assume the former extension of ice-fields over the 

 glaciated areas ; for although I know that there are still 

 dissentients from this fundamental proposition, the cumulative 

 evidence in its favour has been so frequently recapitulated that 

 it would not be justifiable for me to detain you by repeating the 

 arguments. 



It is now, I think, agreed by all who accept this proposition 

 that the ice-sheets of the Glacial Period, though of vast extent, 

 had their northern as well as their southern limits ; the original 

 idea, that they represented the outer portion of a polar ice-cap, 

 having been disproved by more extended researches in the 

 more northerly part of our hemisphere. Moreover, it has been 

 found that these ice-sheets had their origin in the coalescence 

 of masses which spread outward from separate areas ot accu- 

 mulation, acting more or less independently, so that the 

 individual sheets did not all attain their farthest bounds at the 

 same time. But this recognition of independent centres ot 

 glaciation has given sharper prominence to the question whether 

 the glacia' deposits are to be regarded as the product of a 

 single epoch of glaciation, or whether they represent successive 

 epochs of this kind, separated by intervals during which the 

 great ice-sheets temporarily vanished. 



As opinion stands at present, probably most geologists lean 

 to the idea that the glaciation was interrupted Dy at least one 

 interglacial epoch, during which the climate of any particular 

 latitude became not less warm, and perhaps warmer, than it 

 now is. This is the interglacial hypothesis in its simplest form. 

 But it has been frequently pointed out that the criteria depended 

 upon in the recognition of warm interglacial conditions cannot 

 be all assigned to the same horizon, since they recur at different 

 positions in the drift series. Hence it has been claimed that 

 two, three, four, or even five interglacial epochs, with a cor- 

 responding number of separate epochs of glaciation, may be 

 recognised in the glacial sequence. In respect to the number, 

 relative importance, and correlation of these epochs or stages 

 in different countries, or in different parts of the same country, 

 there has been, however, no pretence to agreement among the 

 upholders of the Interglacial idea. 



1906 September i. 



