272 A. GEIKIE ON" THE SUPPOSED 



gives, at a distance, a deceptive resemblance to bedding. This 

 resemblance, however, disappears on examination. 



Numerous other systems of similar joints cut through the mass, 

 precisely as they do through any eruptive crystalline rock. But 

 nowhere have they the character of the divisional planes of a 

 foliated rock, nor do they correspond with any internal arrangement 

 of the component materials in parallel folia*. 



Dr. Hicks lays stress on the fact that, owing to its tendency to 

 split, the rock cannot be dressed for building- or paving-purposes. 

 He proceeds to generalize this fact into a kind of test " in distin- 

 guishing many of the metamorphic rocks from those of igneous 

 origin"f. But surely there is no more familiar structure among the 

 eruptive rocks than their tendency towards multiplied jointing in 

 certain directions. Even in a massive homogeneous granite, where 

 a practised geologist could not detect the least trace of any divis- 

 ional planes, the quarry men will at once show him what they call 

 the " reed" of the rock, along which it will break easily, but across 

 which its fracture is less reliable and definite. Prom this condition 

 every gradation may be traced, especially among the weathered 

 parts, until the rock splits into slabs and might at first be mistaken 

 for a bedded mass. 



A tendency to split in a given direction is therefore no necessary 

 indication of bedding, and need have no connexion with foliation. 

 Had the rock of St. David's been one which might be classed with 

 the gneisses and schists, it would certainly have revealed abundant 

 proofs of foliation — that is, of a crystalline arrangement of its 

 component minerals parallel with the general divisional planes of 

 the rock. Dr. Hicks asserts that " traces of foliation are abun- 

 dant " J. I can only meet this assertion by the statement that my 

 companion and I searched most carefully every exposure of the rock 

 we could find on the ground, and that I have since examined 

 microscopically a series of specimens taken from &11 parts of the 

 ridge, without detecting either on the large or the small scale any, 

 even the most distant, approach to a foliated structure. Many 

 eruptive granites exhibit perfect foliation along certain pegmatite 

 veins ; but even this structure we failed to detect §. 



Between the walls of the joints various decomposition-products 



* It may not be out of place to quote here, in confirmation of our observa- 

 tions, those of Mr. Tawney. He recognized the tendency of the rock at Porth- 

 lisky to split into flaggy and rhomboidal pieces, owing to concealed laminae 

 coated with a thin chloritic lining, and was disposed to look on this structure 

 as bedding ; but he states that " elsewhere it is difficult to say which divisional 

 planes are dominant or less irregular than the others." (Proc. Bristol Nat. 

 Soc. IS". S. vol. ii. pt. ii. p. 117.) 



t Geol. Mag. dec. 2, vol. viii. p. 143 (1881). 



{ Geol. Mag. dec. 2, vol. viii. p. 142. 



§ Dr. Callaway also could find no trace of foliated structure in the crys- 

 talline rock of St. David's, though he searched for it in the principal localities 

 named in Dr. Hicks's papers (Geol. Mag., dec. 2, vol. viii. pp. 94, 237, 1881), 

 and, as he adds, it is not mentioned as existing in any of the microscopic 

 descriptions that have been published of the rock. 



