30 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 19, 



as already intimated, by glaciers, and carried probably, in part, still 

 further by floating ice, before they became subjected to the influences 

 of the currents in question. 



It may be thought that the non-existence of granitic boulders 

 within the mass of the Till is a proof that the transport of the 

 boulders and the Till did not take place at the same time and by 

 the same agency. This argument appears to me conclusive against 

 the opinion that any considerable portion of the Till may have been 

 transported by ice, because there is no reason why a glacier or floating 

 ice should not deposit the smaller detritus together with the blocks 

 with which it may be simultaneously laden. But such would not be 

 the case with currents of water. A current rushing down a subma- 

 rine valley in consequence of an elevatory movement of the central 

 mountains would proceed with diminished velocity as it reached the 

 lower and wider parts of the valley, and especially when it should 

 have escaped into the more open region beyond. It would thus first 

 lose its power of moving the larger, and then of the smaller blocks ; 

 while it would still retain the power of transporting the finer detritus 

 to greater distances. A transient current of this kind, accompany- 

 ing a wave of elevation, could only transport a block of a given mag- 

 nitude a certain distance, as I have elsewhere shown*. Another 

 similar wave at a future time might carry it a certain distance further, 

 and so on for successive waves ; but, if the valleys down which they 

 proceeded were sufficiently long, a continued succession of these 

 waves might be necessary to transport the blocks to that more open 

 region in which these waves would deposit successive layers of finer 

 matter, and on reaching that more open space they would be left on 

 the finer matter previously deposited there. According to this process, 

 wherever the blocks and finer sediment should be found together, the 

 finer sediment would invariably form the lowest stratum. In the 

 southern Highlands, the limits within which there is any considerable 

 number of blocks scarcely extend to the boundaries of the Till, and 

 blocks are scarcely ever, I believe, found imbedded in it. Mr. Griffith 

 has informed me that he believes the limestone boulders of Ireland 

 to repose in many instances on the finer detrital matter, and M. 

 d'Archiac cites several observers who testify that the same arrange- 

 ment obtains very generally in the north of Europef . It will pro- 

 bably be found to be a very general character of most masses of drift. 

 The explanation here given of it does not at all interfere with the 

 theories of the transport of the larger and more angular blocks by 

 ice, for it is intended to apply more especially to those smaller and 

 more rounded blocks which are usually found at considerable di- 

 stances from the original sources whence they have been derived. 



* Transactions of the Phil. Soc. of Cambridge, vol. viii. Part 2. 

 t See also Murchison's * Russia.' 



