98 THE STRUCTURE OF THE [Ch. VI. 



together, and which appear to subsist in the same mass, in 

 conjunction with various cuboidal and prismatic arrange- 

 ments. 



The fissures or joints which occur in the upper part of 

 granitic rocks, are produced either by direct exposure to the 

 atmosphere, or by the percolation of rain water ; and it has 

 been stated, that they unfold the form of the concretions of 

 which the aggregate mass is composed, just as the funda- 

 mental forms of a crystalline mass are displayed by the partial 

 action of a chemical solvent. And this analogy is confirmed 

 by the fact, that in both cases the same result may be 

 obtained by mechanical means ; thus, large blocks of granite, 

 or the mass of this rock, at depths where the joints are no 

 longer visible? may be cloven into cuboids, by means of 

 several wedges applied on the same line ; and in this manner 

 all the granitic rocks may be divided into blocks correspond- 

 ing in form to those developed by the action of the elements. 

 According to the case already related, concerning the schistose 

 granite of Arran, by Dr. Macculloch, this does not always 

 hold good; but; this discrepancy may, perhaps, be attributed 

 to the imperfection of the mechanical power which he em- 

 ployed, conjoined with the refractory nature of the rock 

 operated on. The geologist would not readily arrive at the 

 fact, that common granite can be separated into cubes ; for 

 with his hammer and wedges he can only break off irregular 

 fragments : but the quarryman, by first boring several holes 

 on a given line, at an equal distance from each other, and 

 then alternately striking wedges placed therein, can cleave 

 the granite into regular quadrangular masses ; and, in like 

 manner, slate may be raised in large tabular slabs by the 

 simultaneous application of many broad wedges, whereas the 

 geological tools would only shiver the slate into foliated frag- 

 ments. 



That the cleavage of granite into regular forms depends on 

 the structure of this rock, is proved by its only taking place 

 in certain directions corresponding therewith. The workmen 



