Of a New Granite Tract in Arran. 21 



are observable. These are highly interesting, and clearly indicate the 

 intrusion of the granite subsequently to the formation of the conglo- 

 merate. The base of this conglomei-ate is a coarse sand, and the 

 imbedded fragments sandstone, quartz, and granite. The base is highly 

 indurated, and assumes a porphyritic structm'e ; the sandstone is ren- 

 dered crystalline, and the quartz has been fused, and often converted 

 into a substance resembling poreellanite. The fragments of granite 

 are of an eUiptic form, less rounded than the quartz, and are exactly 

 like the adjoining mass of granite in structm'e and component parts. 

 Wlience have these granite fragments been derived ? From the body 

 of fine granite among the northern mountains, or from the adjoining 

 mass itself? Mineral structure does not enable us to determine — 

 the two rocks are so similar. If from the former som'ce, then we 

 must conclude that the granite of the interior was elevated so as 

 to be exposed to disintegrating causes, while the conglomerate was 

 forming ; in which case granite fragments ought to occur abundantly 

 in the sandstone conglomerates ; but this is not found anywhere in 

 Arran ; — a fact noticed by all observers. Even here the fragments occur 

 only in close proximity to the granite itself. Must we not then rather 

 suppose that pieces of the granite adjoining, when this rock was 

 erupted in a fluid or semi-fluid state, were injected among the outer 

 strata of the conglomerate, also fused by the contact, and so became 

 imbedded in these strata only ? 



Granite, then, occurs in Arran in three disconnected tracts, and the 

 question remains, are these of three distinct ages, or were they erupted 

 simultaneously, so as to pierce through the three formations during one 

 and the same period of disturbance ? The latter is by far the most probable 

 supposition, because we find ; first, that while the granite of the nucleus, 

 that is, of the interior mountains, everywhere pierces through, and 

 alters the enveloping slate-band ; this slate-band, in some places where 

 it is very narrow, has also altered the old red sandstone in contact with it, 

 having been itself in fusion from contact with the molten granite ; and, 

 secondly, that the actual position of the old red sandstone and carbo- 

 niferous limestone along the Corrie shore, indicates great upheaval and 

 disturbance by protruding masses of granite advancing in that direction 

 from the nucleus. The entire series of sedimentary strata in Arran 

 was therefore deposited prior to the intrusion and elevation of the 

 granitic masses ; and the Craig- Dhu and Ploverfield tracts were most 

 probably formed simultaneously with that of the northern mountains. 

 This question will be found discussed at greater length in the 

 second edition of a pamphlet on Tlte Geology of Clydesdale and the 

 Clyde Islands. 



