40 GEOLOGY OF ARRAN. 



which they must have passed across deep and narrow glens. 

 They are found also isolated on the Holy Isle, which is 

 separated from the mainland by a wide bay and two deep, 

 navigable channels. Blocks of the coarse-grained variety are 

 much more numerous than those of the fine. The latter, 

 indeed, are in a great measure limited to the tracts on which 

 Glen lorsa, the principal seat of this variety, opens towards 

 the south ; and this fact, in connection with the more sparing 

 distribution of the blocks along the northern coast, on which 

 but one glen with a narrow opening debouches, than in other 

 parts of the island, shews that, though the dispersion has 

 been quaquaversal, it has been to a large extent determined 

 by the direction of the valleys. Dr. MacCulloch, who has 

 noted the leading facts regarding the dispersion of the granite 

 blocks with great accuracy, though imperfect in many of his 

 details, closes his account with the following observations : 

 " None of the blocks have the marks of a distant origin ; all 

 have the characters of the granites of the adjoining moun- 

 tains, characters sufficiently distinct from those of almost all 

 the granites of Scotland. . . . No situation, perhaps, has 

 been pointed out where the origin of the travelled blocks is 

 more obvious, or their new position more difficult to compre- 

 hend, without assuming considerable revolutions of the sur- 

 face of the land over which they have passed. . . . The 

 compact and solitary position of the fixed mass of granite, 

 the identity of the materials of this mass with that of the 

 travelled stones, the gradual diminution of these as they 

 recede from the parent rock, and the insulated position of 

 the whole, render their origin indubitable, and present to the 

 geologist a spot, on the changes of which he may speculate, 

 with the certainty that he has before him a set of incontro- 

 vertible data from which to reason." (Ut sup., p. 341.) 



This passage places in a clear light the conditions of the 

 problem and the difficulties attending it. The author does 

 not, however, propose a solution of the difficulties, nor does 

 he enter into any theoretical discussion. His account of the 



