Ch. XXXIV.] OF THE ISLE OF ARRAN. 729 



At and near the point of contact the base of the conglomerate of 

 the Old Red consisting of sandstone is rendered crystalline, and frag- 

 ments of granite of an elliptical form and of the same mineral struc- 

 ture as the adjoining mass of fine granite are included in the sandstone 

 or conglomerate. It has been already stated that no pieces of granite, 

 rounded or angular, occur elsewhere in the Old Red, and as they are 

 only found here in close proximity to the crystalline and intrusive 

 rock, Dr. Bryce supposes that they were injected into the strata in a 

 fluid or semi-fluid state. I have seen a similar junction in Caithness, 

 of which Sir R. Murchison in 1827, and again in 1828 jointly with 

 Professor Sedgwick, has given a faithful description. Close to the 

 point of contact of certain oolitic sandstones, shales, and limestones in 

 the Caithness cliffs, a breccia occurs containing granite fragments 

 mixed with those of the invaded rock. The granite, they say, appears 

 as if it had been mechanically driven in among the shattered and 

 altered strata.* In the coarse-grained granite of the northern nucleus 

 trap-dikes of pitchstone and basalt are numerous, but dikes are com- 

 paratively rare in the fine-grained granite, and were even supposed to 

 be entirely wanting until three or four, consisting of basalt and green- 

 stone, were discovered by Br. Bryce, running north and northwest. 

 It seems therefore fair to infer, as Prof. Ramsay has done, that many 

 of the dikes penetrating the older granite are cut off at the junction 

 of the newer or fine-grained variety in the manner expressed at h, c, d, 

 fig. 754, though no such cutting off has been actually observed. We 

 may also feel assured that some of the dikes traversing the fine must 

 also penetrate the coarse-grained granite, as Dr. Bryce infers, 

 although, as yet, he has not observed the actual passage of any one 

 from the newer to the older rock. There can be scarcely a doubt 

 that the fine-grained variety of nucleus and the similar rocks of 

 Ploverfield and Craig-Dhu are of contemporaneous date, and they 

 seem to be more modem than all the formations of Arran, except the 

 overlying trap (No. 6) and its associated dikes. But 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, and which 

 it alters at the point of contact. 



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

 derived 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, 

 although 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 the wreck of preexisting rocks occurring in the 

 immediate vicinity. The total absence therefore of granitic pebbles 

 has justly been a theme of wonder to those geologists who have 



* Geol. Trans., Second Series, vol. ii. p. 353, and vol. iii. p. 132 



