ARRAN. GEOLOGY. GRANITE. 347 



quence, the Glen of Catcol as well as the other neighbour- 

 ing valleys. It is in the upper part of this valley that the 

 structure in question is most conspicuous. Here the rock 

 is occasionally prismatic, and on a much more minute 

 scale than as it occurs under that form in Caime na cail- 

 lich and other places on the eastern side of the mountains ; 

 since the prisms, which present a varying number of 

 angles, frequently do not exceed a few inches in diameter. 

 It is also much more often schistose. The laminse into 

 which it exfoliates, vary in thickness, but may be found so 

 thin as not to exceed the tenth of an inch ; while they may 

 sometimes be raised in succession, and of considerable 

 dimensions, from the same surface of a solid mass ; indi- 

 cating that the whole will hereafter, under exposure of 

 sufficient duration, be resolved into similar forms. This 

 schistose structure is irregularly interspersed together with 

 the prismatic, and, in some rare instances, they will be 

 found united in the same block ; a circumstance, as 

 will hereafter be shown, of frequent occurrence in the 

 porphyry of this island. 



With respect to the composition of this granite I must 

 remark, that it contains but occasional scales of black 

 mica, and that it is principally composed of an intimate 

 and minute admixture of quartz and felspar, with small 

 crystals of hornblende sparingly dispersed throughout. It 

 is consequently of a very light grey colour. The schistose 

 structure therefore, depends in no wise on the mica, and 

 presents no analogy to that of gneiss; the rock being 

 perfectly and equally granular throughout, in a degree 

 indeed equal to that of many sandstones. Nor, when 

 the exfoliation has taken place, do the surfaces show any 

 marks of decomposition; the natural freshness of the 

 rock remaining nearly such as it would be after a forcible 

 fracture. I may add, that the blocks which seem likely 

 to undergo this change, and even those in which it has 

 actually commenced, show no symptoms of future and 

 similar exfoliation; and that all mechanical attempts to 



