162 Trof. J, Young — Deposits preserved under " Till.'" 



And yet in the tables of strata geologists almost invariably adopt 

 the linear arrangement of strata, and have a tendency to take the 

 maximum thickness of each group of rocks. Thus we group certain 

 rocks as under : 



Chalk 



Upper Greensand 



Gault 



Lower Greensand 



Wealden 



Purbeck. 

 Professor Young, however, in his Address, thought it highly probable 

 that " the Lower Greensand is contemporaneous with part of the 

 Chalk, so were parts of the Wealden ; nay, even of the Purbeck a 

 portion must have been forming while the Cretaceous sea was 

 gradually deepening southward and westward." Our earth's history, 

 indeed, is not like the simple history of one kingdom, but more 

 closely resembles the contemporaneous history of several provinces. 



{To be concluded in our next Number.) 



III. — What must be Explained before the PRESERVATioisr of 



Deposits under Till is Explained. 



By Professor John Young, M.D., Glasgow University. 



AS I am one of those who find difficulty in understanding the 

 preservation of stratified deposits beneath the Till, I would 

 like to point out to Mr. J. Geikie that he has not correctly stated 

 the difficulty in his paper (Geol. Mag. Feb. 1878) on "The Pre- 

 servation of Deposits under Till or Boulder-clay." The difficulty 

 really lies in the explanation of how the Till itself was deposited : 

 were that clear, the preservation of beds beneath it might be at once 

 intelligible. Admitting the passage of an ice-sheet across the country, 

 admitting further that the Till is the moraine profonde of such an 

 ice-sheet, the accumulation of 100 feet of Till beneath the ice-sheet 

 is a phenomenon which has not been explained, and cannot be passed 

 over as unimportant. A moving mass of ice, 2000 feet thick, 

 seems a formidable agent of erosion ; yet where I now write nearly 

 100 feet of Till intervene between the surface and the Carboniferous 

 rocks, which, where exposed, shows striations having the usual 

 compass bearing for this neighbourhood. The greater portion of the 

 Till hereabouts is derived from the Carboniferous rocks of the dis- 

 trict, but numerous boulders of granite, felstone, schist, and of Old 

 Eed Sandstone sedimentary and volcanic rocks, show that the ice 

 had brought materials from a wider area. The limits of the 

 Carboniferous series are about twelve miles to the west and north- 

 west of Glasgow, that being the direction whence the ice came. 

 As the ice of a modern glacier pushes the results of erosion before 

 it, we may conclude that the old ice-sheet with a pressure of about 

 1000 lbs. to the square inch did the same ; indeed, Mr. Geikie has 

 already accepted this " idea as to how an ice-sheet would behave," 

 but without explaining how the alternate exposure and con- 



