G. W. Bulman — Glacial Geology. 345 



that, in the absence of sections, there is here no evidence of an 

 interglacial epoch between the periods of the chalky and purple clays. 



Again, glancing at Mr. Jukes-Browne's map, one is inclined to ask, 

 why, on the hypothesis that the formation of the purple clay ivas 

 subsequent to that of the Chalky, none of the former was laid down 

 OH the latter over all that region west of the chalk Wolds where it 

 occurs ; while at the same time it does occur further west and north 

 away from the chalky boulder clay ? And why, on the other hand, 

 the chalky clay is not found beneath the purple on the east of the 

 Wolds ? 



As regards the division between the purple and Hessle clays, 

 Mr. Jukes-Browne shows that the sands and gravels separating 

 them graduate into both lower and upper clays ; the sand beds 

 containing patches of boulder-clay, and the latter beds of sand in 

 the usual way. He concludes finally that there is no break between 

 the clays. 



The presence of Cyrena fluminalis, noted by Mr. Wood in the 

 Hessle beds, may be taken as an indication that the climate was 

 ameliorating as the ice gradually disappeared. 



Prof. Geikie points to the Interglacial beds, of other countries as 

 additional evidence of mild interglacial periods in our Northern 

 hemisphere — he claims it, in fact, from every country yet examined : 



" In every country where the glacial deposits have been studied, we 

 have clear proof of a mild interglacial period having supervened." ^ 



The intercalated beds of Switzerland are of special interest, con- 

 sisting as they do of seams of lignite from two to five and some- 

 times even twelve feet thick, and which have been worked for fuel. 

 They are made up of the remains of peat-forming plants, and rest on 

 sand and clay, beneath which is the grund-morane. Eemains of 

 Pines, Oaks, Birches, Larches, etc., occur in these lignites, from 

 which Prof. Heer, has inferred that the climate, at the time of its 

 formation, was the same as that of Switzerland at the present day. 



The inference drawn by Prof. Geikie is, " that the great mer de 

 glace eventually vanished from the low grounds, and the glaciers 

 shrunk back again into the deep mountain-valleys. The climate 

 grew as mild as it is at present. Oaks, Pines, and other trees 

 overspread the ground, and many large animals became denizens of 

 Switzerland. That this condition of things must have endured for 

 a long time no one can doubt. Nor could the change from the 

 intense glacial climate of the great mer de glace have been other 

 than gradual. The glaciers would slowly retire, and many ages 

 would elapse before the condition became such as to induce the 

 growth of Oak-trees. After the genial climate that nourished these 

 trees had lasted for untold centuries, the cold again increased. 

 Slowly the glaciers crept down the valleys. Little by little, year 

 by year, they continued to advance, until at last, escaping from the 

 mountain- valleys, they deployed upon the low grounds. And now, 

 encroaching upon, and eventually occupying the basins of the 



^ Great Ice Age, p. 491. 



