Correspondence — Rev. A. Irving. 191 



that no agency other than moving-ice could produce the effects 

 observed, perhaps I may venture to ask for space for a few lines. 

 J wish to point out that there appears to be considerable conlusion 

 of thought on this matter. 



The rock-fragments are described now as "pebbles," now as 

 " boulders," which are surely representatives of very different sets of 

 mechanical forces. The term "pebble" is certainly a misnomer if 

 applied to the two specimens which were exhibited at Birmingham. 

 I saw and handled them both. The felsitic block had portions of 

 its original surface somewhat smoothed, the sharper lines of its 

 fracture somewhat rounded off in a fashion suggestive of the way in 

 which the more flinty sarsens of this district acquire a certain degree 

 of polish. The striated " facets " on this block are certainly difficult 

 of explanation by reference to glacial agencies : they seemed to me 

 very different from the ice-striations of blocks, of which I have had 

 rather extensive observation in Alpine regions; nor can the necessary 

 retention of the block in a fixed position in resistance to great and 

 long-continued pressure be reconciled with the known physical 

 properties of ice (see Q. J. G. S. February, 1883, pp. 62 et seq.). 



The other block exhibited was more of a basaltic character, and 

 was no doubt a slightly water- worn fragment of a basaltic column, 

 as Prof. Carvill Lewis pointed out in the discussion at Birmingham. 

 I do not recollect that this block was striated. 



The question as to how the blocks came into their present position 

 is entirely distinct from that of the agencies by which their present 

 surface-character was given to them ; and we have no right to assume 

 that these agencies acted upon them simultaneously. On the contrary, 

 Mr. Oldham's description ' of the beds in which they occur affords, I 

 think, convincing evidence to show that these facets and striations 

 were produced by some agency or other prior to their deposition in 

 the beds in which they now occur. This disposes at once of that 

 writer's objection to the landslip theory (miscalled " soil-cap move- 

 ment"), which I urged as the most likely explanation of the 

 phenomena. In the discussion I referred to the objections of so 

 excellent a physical geologist as Prof. Heim'^ of Zurich, to which I 

 have on a previous occasion drawn attention in the pages of this 

 Magazine (Decade II. Voh X. pp. 160 et seq.), where it was also 

 pointed out that the polishing and striations of the surfaces of 

 fragments of rock by the slow grinding movement, which often 

 goes on for years, was worthy of some consideration from their 

 resemblance to some of the effects of glacial action. As I fail to 

 see that this explanation has been met as yet by any insuperable 

 objection, it will not be from mere obstinacy if 1 still adhere to it 

 as upon the whole the most probable. 



Mr. Oldham remarks on the origin of the name "Olive Group," 

 and then a few lines further on says there is not "any sign of 

 carbonaceous matter in the bed." Has chemical analysis decided 

 this ? A. Irving. 



"Wellington College, Berks, 

 January Sth, 1887. 



1 Vide Geol. Mag. January, 1887, p. 32. 2 ((XJeber Bergstiirse." 



