74 



claystone. But at a high level on the west, south, and east sides 

 of the plateau, the granite is seen to rise through a coarse conglo- 

 merate ; and numerous contacts are observable. These are highly 

 interesting, and clearly indicate the intrusion of the granite sub- 

 sequently to the formation of the conglomerate. The base of this 

 conglomerate is a coarse sand, and the imbedded fragments sand- 

 stone, quartz, and granite. The base is highly indurated, and assumes 

 a porphyritic structure; the sandstone is rendered crystalline, and 

 the quartz has been fused, and reconsolidated into a substance 

 resembling porcellanite. The fragments of granite are of an elliptic 

 form, less rounded than the quartz, and are exactly like the adjoin- 

 ing mass of granite in structure and component parts. Whence 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 source, 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 abun- 

 dantly in the sandstone conglomerates ; but this is not found any- 

 where 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 ? 



52. Granite, then, occurs in Arran, in three disconnected tracts, 

 amid slates in the northern mountains, in the old red sandstone of 

 Craig-Dhu, or at the junction of this rock with the carboniferous 

 strata, and also amidst these carboniferous strata at Ploverfield. In 

 each of these positions it is clearly posterior to the rock which 

 encircles it ; for it is intruded among these sedimentary deposits, and 

 produces a marked alteration upon them along the planes of contact. 

 We have, therefore, now to consider the question of age. Are the 

 three granites of three distinct ages corresponding with those of the 

 strata among which they intrude ? or were they erupted simultane- 

 ously, so as to pierce through the three formations during one and 

 the same period of disturbance ? In other words and this view 

 narrows the question since the Ploverfield granite is clearly of later 

 origin than the sandstones of Windmill Hill, and the shell lime- 

 stones subordinate to them, were the granites of Goatfell and Craig- 

 Dhu erupted at the same time with it ? or does their injection 



