584 AGE OF THE GRANITES. [Oh. XXXIV 



form of dikes, or in dense masses from 50 to 700 feet in thickness 

 overlying the strata (No. 4). They sometimes pass into syenite of so 

 crystalline a form, that it may rank as a plutonic formation ; and in one 

 region, at Ploverfield, in Glen Cloy, a fine-grained granite (6 a) is seen 

 associated with the trap formation, and sending veins into the sandstone 

 or into the upper strata of No. 4. This interesting discovery of granite 

 in the southern region of Arran, at a point where it is separated from 

 the northern mass of granite by a great thickness of secondary strata 

 ' and overlying trap, was made by Mr. L. A. Necker of Geneva, during 

 his survey of Arran, in 1839. We also learn from late investiga- 

 tions by Professor A. C. Ramsay, that a similar fine-grained granite (No. 

 6 h) appears in the interior of the northern granitic district, forming the 

 nucleus of it, and sending veins into the older coarse-grained granite 

 (No. 2). The trap dikes which penetrate the older granite are cut off, 

 according to Mr. Ramsay, at the junction of the fine-grained. 



It is not improbable that the granite (No. 6 b) may be of the same 

 age as that of Ploverfield (No. 6 a), and this again may belong to the 

 same geological epoch as the trap formations (No. 5). If there be any 

 difference of date, it would seem that the fine-grained granite must be 

 newer than the trappean rocks. But, on the other hand, the coarser 

 granite (No. 2) may be the oldest rock in Arran, with the exception of 

 the hypogene slates (No. 1), into which it sends veins. 



An objection may perhaps at first be started to this conclusion, de- 

 rived from the curious and striking fact, the importance of which was 

 first emphatically pointed out by Dr. MacCulloch, that no pebbles of 

 granite occur in the conglomerates of the red sandstone in Arran, though 

 these conglomerates are several hundred feet in thickness, and lie at the 

 foot of lofty granite mountains, which tower above them. As a general 

 rule, all such aggregates of pebbles and sand are mainly composed of 

 Ike wreck of pre-existing rocks occurring in the immediate vicinity. 

 The total absence therefore of granitic pebbles has justly been a themo 

 of wonder to those geologists who have successively visited Arran, and 

 they have carefully searched there, as I have done myself, to find an 

 exception, but in vain. The rounded masses consist exclusively of 

 quartz, chlorite-schist, and other members of the metamorphic series; 

 nor in the newer conglomerates of No. 4 have any granitic fragments 

 been discovered. Are we then entitled to affirm that the coarse-grained 

 granite (No. 2), like the fine-grained variety (No. 6 a), is more modern 

 than all the other rocks of the island ? This we cannot assume at 

 present, but we may confidently infer that when the various beds of 

 sandstone and conglomerate were formed, no granite had reached the 

 surface, or had been exposed to denudation in Arran. It is clear that 

 the crystalline schists were ground into sand and shingle when the strata 

 No. 8 were deposited, and at that time the waves had never acted upon 

 the granite, which now sends its veins into the schist. May we then 

 conclude, that the schists suffered denudation before they were invaded 

 by granite ? This opinion, although not inadmissible, is by no means 



