94 THE STRUCTURE OF THE [Ch. VI. 



found to be again subdivided into lesser squares, as frequently 

 occurs in the cliffs of the Land's End district : or, if the 

 quadrangular blocks be freely exposed to the action of the 

 elements, they are gradually converted into spheroids of 

 various configuration, by the destruction of the solid angles, 

 as well exemplified in the tors and logan-stones which crown 

 the most elevated hills in granitic districts. " This pecu- 

 liarity of structure oftentimes produces vast piles of blocks, so 

 regular in their form and arrangement, that they convey a 

 striking resemblance to a gigantic edifice in a state of ruin ; 

 and lead us to do homage to the works of Nature, by con- 

 trasting them with the petty operations of human skill." 



When the structure of the granitic rocks is more par- 

 ticularly examined, it will be found that the concretions, into 

 which they are divided by the intersecting systems of fissures 

 or joints, present various kinds of forms : thus, according as 

 the horizontal joints approach nearer or recede farther from 

 each other than in the cube, tabular or prismatic masses are 

 produced ; and as the vertical joints decline more or less from 

 the perpendicular, different modifications of the rhomboid are 

 formed. The figure of the concretions appears in some 

 measure to be connected with the composition of the rock. 

 For example, the common granite (as defined in the pre- 

 ceding pages) exhibits the most perfect cuboidal concretions ; 

 the fine-grained granite and protogine disintegrates into 

 tabular masses; shorlaceous granite and shorl-rock often 

 assume the prismatic form ; and the rhomboidal structure is 

 characteristic of all the kinds of felsparite, which, however, is 

 not the simple form of this rock, for it separates diagonally 

 into various angular concretions, which renders it a bad 

 building material. These facts are too few to justify a gene- 

 ralization, and are balanced by many exceptions ; but which, 

 it must be confessed, have not been subjected to a minute and 

 circumstantial scrutiny : they are, however, sufficient in 

 number to invite more attention than has been hitherto 

 bestowed on this subject. 



