PART 11. CHAPTER XXIV. 



293 



Age of the Granite of the Isle of Arran. 



than the crystalline schists ; but can we decide them to be like- 

 wise younger than the secondary sandstones ? 



Now it is a curious and most striking fact, that no pebbles of 

 granite have ever been found in the conglomerates of the red 

 sandstone in Arran, although careful search has been made for 

 them by many geologists ; and although puddingstones in gene- 

 ral are chiefly made up of fragments of older rocks of the imme- 

 diate vicinity. The total absence of such pebbles has justly been 

 a theme of wonder to those who have visited Arran, and have 

 seen that the conglomerates are several hundred feet in thick- 

 ness, and that they occur at the base of the granite mountains, 

 which tower above them in far bolder and more picturesque 

 forms than those of similar composition in other parts of Scot- 

 land. We may at once infer, with confidence, that when the 

 sandstone and conglomerate were formed, no granite had reached 

 the surface, or had been exposed to denudation in this region : 

 the crystalline schists were ground into sand and shingle when 

 these puddingstones were accumulated, but the waves had never 

 acted upon the granite which sends its veins into the schist. 

 Are we then to conclude, that the schists suffered denudation 

 before they had been invaded by granite? This opinion, al- 

 though it cannot be disproved, is by no means fully borne out 

 by the evidence. At the time when the red sandstone was 

 formed, the metamorphic strata may have formed islands in the 

 sea, as in Fig. 294., over which the breakers rolled, or from 



Fig. 294, 



which torrents and rivers descended, carrying down gravel and 

 sand. The plutonic rock (b) may have been previously injected 

 at a certain depth below, and yet may never have been exposed 

 to denudation. 



As to the time and manner of the subsequent protrusion of 

 the hypogene rocks in Arran, these are questions into which I 

 have not space to enter at present : I shall merely observe, that 

 those crystalline rocks may have been thrust up bodily, in a 

 sohd form ; and it is clear that, during or since the period of 

 their emergence, they have undergone great aqueous denuda- 

 tion. This action is confirmed by three distinct kinds of proofs ; 

 1st, The occurrence of scattered pebbles and huge erratic blocks 

 of granite and schist over the surface of Arran and the adjacent 



