102 '- " f ;T&E STRUCTURE OF THE [Ch. VI. 



cular faces which, being traversed by upright lines, represent 

 vast piles of columns : but these sections cross the beds at a 

 considerable angle, and their nature may be easily detected 

 by a close inspection of the mass, when they will be seen to 

 result from the rhomboidal concretions being so arranged 

 that one of the three parallel systems of planes of which they 

 are formed, has a perpendicular position. This prevalence of 

 the rhomboidal structure, in the cliffs of this district, is an 

 interesting fact, because the granite is here always at or near 

 the junction with the slate, and generally exhibits such a 

 variation in its mineral composition from the central masses 

 as characterises this rock when it comes into contact with the 

 primary schists, and on which account it is often difficult to 

 decide whether the rocks at the junction be granite or slate, 

 without putting them to the test of the hammer. 



Now in all the cases just quoted, of the bearings of granitic 

 layers or beds, in France and Ireland, the same rhomboidal 

 structure prevails, as indicated by the beds dipping at various 

 angles ; and the granite, in all these places, occurs near the 

 junction with the slate. It becomes, therefore, an interesting 

 enquiry to ascertain, if possible, the limits of this structure ; 

 and under what circumstances the granite assumes a regular 

 cuboidal structure. It is well known that the central part of 

 a mass of granite is not wholly of the latter kind, because it is 

 traversed by beds of eurite, felsparite, and other granitic com- 

 pounds, which are composed of rhomboidal concretions. It 

 will, perhaps, be found that cuboidal blocks are not so 

 common pr characteristic of granitic rocks as is generally 

 assumed. 



Before concluding this topic it may be observed, that the 

 granitic beds necessarily dip at several angles, according to 

 the kind of rhomboids of which they are composed, and to the 

 position in which they are placed : thus, in granitic cliffs, the 

 rock will be commonly found to present a perpendicular face, 

 and two inclined ones ; one of the latter dipping at a very 

 elevated angle, and the other at an angle of less than 4-5 : 



