162 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 26, 



attains a thickness of 100 feet, marine shells identical with those 

 found by myself at Bnshbury and Oxley Manor have been detected 

 by Mr. Beckett, at Mr. Sparrow's colliery of Portobello. This fact is 

 valuable as a determination of the relative position in which we shall 

 be justified in placing these shell-bearing clays in the drift- deposits 

 of Staffordshire. 



2. 071 a Split Bouldee in Little Cumbea, Western Isles. 

 By James Smith, Esq., F.E.S., F.G.S., of Jordan HiU. 



Split erratic blocks are of frequent occurrence in Switzerland. The 

 only explanation of this phenomenon which I have met with is that 

 of M. Charpentier, in his " Essai sur les Glaciers." Speaking of the 

 blocks, he says, " Quelques ims sent fendus, mais la direction des 

 fentes prouve jusques a Tevidence que les ruptures sent le resultat 

 d'une chute et nullement d'un choc horizontal " (p. 180). M. Char- 

 pentier offers no conjecture as to the height from whence the blocks 

 could have fallen ; but where there is no superincumbent precipice of 

 rock near, it must have been from one of ice. Indeed, I may say that I 

 obtained proof that such was the case ; for upon examining the frag- 

 ments which lay at the foot of the escarpment of ice which terminates 

 the Glacier of Grindelwald, I observed one which, from the freshness 

 of the fracture, I concluded must have fallen very shortly before my 

 visit, and obviously from the sui^face of the glacier. 



Such blocks occur occasionally in the basia of the Clyde, in situa- 

 tions where there is no adjoining height from which they could have 

 fallen, — a circumstance which I can only account for by supposing the 

 former existence, in the same localities, of ice in the shape of glaciers, 

 icebergs, or coast-ice. I may add that some of the split boulders 

 are also scratched, exhibiting additional proofs of glacial action. 



To one of these blocks I wish to call the attention of the Society, 

 on account of the pecuharity of the circumstances of its present 

 position. There is on the west coast of Scotland a well-marked cliff 

 and terrace, indicating an elevation of about forty feet above the 

 present sea-level ; and, from the amount of solid rock which has been 

 removed by the washing action of the sea, we may form some con- 

 ception of the prodigious lapse of time during which the sea-level was 

 stationary at that height. 



This is nowhere better seen than in the Islands of Great and 

 Little Cumbra. The larger island is composed of red sandstone, 

 traversed by trap- dykes ; the smaller one is composed entirely of trap. 

 The trap of the dykes, from its greater hardness, has been worn away 

 more slowly than the sandstone ; hence their projection from the 

 sandstone cliff; hence also the greater breadth of the terrace in 

 (jlreat Cumbra than in that of the trap of the smaller island. 



The terrace in Little Cumbra, formed by the wasting action of the 

 sea at right angles with the coast-line, has been subsequently ground 

 down and scratched by a force acting parallel to it and the ancient 

 cliff; and it is upon this that the blocks in question must have fallen. 



