79 



differences of mineral structure, or aggregation of parts, being often 

 seen in granites certainly cotemporaneous. 



5. But as the coarse grained granite cannot with certainty be pro- 

 nounced newer than the carboniferous strata, while the Craig-Dhu 

 and Ploverfield undoubtedly are so, then we may have two ages 

 for these outbursts one for that of the granite of the nucleus, and 

 another for that of these latter. 



6. The constant character of the fine grained granite of the inte- 

 rior, through a considerable area, and its subordinate position to the 

 coarse, point to a later origin than that of the latter ; while its almost 

 perfect identity in structure and arrangement of parts with the other 

 two granites render very probable the cotemporaneity of these three, 

 and their posteriority to the coarse grained variety. This conclu- 

 sion, which is extremely probable, and to which we chiefly incline, 

 would be greatly strengthened by finding decided veins of the fine 

 variety penetrating the coarse, along the line of junction. The mere 

 occurrence of veins of fine grained granite amid the coarse variety is 

 insufficient. Their connection with the inferior masses, and altera- 

 tions at the line of contact, similar to those at the slate junctions, 

 are the points to be determined. Already, we think, sufficient 

 evidence has been set forth (Art. 46, p. 88), to show that this fine 

 grained granite is not the newest rock in the island, but is older than 

 the traps and porphyries. To a short history of these we now pro- 

 ceed. 



TJie Trappean Rocks. 



55. Arran is extremely rich in rocks of this class ; most of the 

 known species occur, and also those numerous varieties by which 

 these graduate into one another. They form great overlying masses, 

 capping the sandstone of the southern plateau, and rising into the 

 highest hills of this division of the island. They are interposed 

 amid the sedimentary deposits, in huge sheets or beds conformable 



Relation of the Igneous and Sedimentary Rocks. 



to the stratification, and cut through all the rocks alike, from the 

 lowest to the highest, in vertical or slightly inclined dikes, which 

 range continuously across great horizontal distances, in one uniform 

 direction. These dikes are never observed to wedge out downwards ; 

 and no doubt they descend into the hypogene depths, where sheets 



