586 



GRANITES OF ARRAN. 



[Ch. XXXIV. 



fully borne out by the evidence. For at the time when the Old Red 

 sandstone originated, the metamorphic strata may have formed islands 

 in the sea, as in fig. 703, over which the breakers rolled, or from which 



Fig. 703. 



torrents and rivers descended, carrying down gravel and sand. The 

 plutonic rock or granite (b) may even then have teen previously in- 

 jected 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 coarse- 

 grained granite (No. 2), this rock may have been thrust up bodily, in a 

 solid form, during that long series of igneous operations which produced 

 the trappean and plutonic formations (Nos. 5, 6 a, and 6 b). 



We have shown that these eruptions, whatever their date, were poste- 

 rior to the deposition of all the fossiliferous strata of Arran. We can 

 also prove that subsequently both the granitic and trappean rocks under- 

 went great aqueous denudation, which they probably suffered during 

 their emergence from the sea. The fact is demonstrated by the abrupt 

 truncation of numerous dikes, such as those at c, d, e, which are cut off 

 on the surface of the granite and trap. The overlying trap. also ceases 

 very abruptly on approaching the bouudaiy of the great hypogene 

 region, and terminates in a steep escarpment facing towards it as at f, 

 fig. 702. When in its original fluid state it could not have come thus 

 suddenly to an end, but must have filled up the hollow now separating it 

 from the hypogene rocks, had such a hollow then existed. This neces- 

 sity of supposing that both the trap and the conglomerate once extended 

 farther, and that veins such as c, d, fig. 702, were once prolonged farther 

 upwards, prepares us to believe that the whole of the northern granite 

 may at one time have been covered by newer formations, under the 

 pressure of which, before its protrusion, it assumed its highly crystalline 

 texture. 



The theory of the protrusion in a solid form of the northern nucleus 

 of granite is confirmed by the manner in which the hypogene slates 

 (No. 1), and the beds of conglomerate (No. 3), dip away from it on all 

 sides. In some places indeed the slates are inclined towards the granite, 

 but this exception might have been looked for, because these hypogene 

 strata have undergone disturbances at more than one geological epoch, 

 and may at some points, perhaps, have their original order of position 

 inverted. The high inclination, therefore, and the quaquaversal dip of 

 the beds around the borders of the granitic boss, and the comparative 

 horizontality of the fossiliferous strata in the southern part of the island, 

 are facts which all accord with the hypothesis of a great amount of 

 movement at that point where the granite is supposed to have been 



