378 Dr. Boase's Inquiry into the Nature 



instead of equably affecting the whole mass, especially whilst 

 subjected to immense pressure. But we know that such 

 fissures, at or near the surface of the earth, can be, and in 

 numerous cases probably have been, developed by the perco- 

 lation of water and other atmospheric causes, acting in the 

 direction of previously existing joints : thus unfolding the 

 forms of the rock-concretions, just as the fundamental figures 

 of a crystalline mass are displayed by the partial action of a 

 chemical solvent; an analogy confirmed by the fact, that large 

 blocks of granite, or the mass itself in which the joints are not 

 visible, may be mechanically divided into quadrangular por- 

 tions, corresponding in form to those produced by the action 

 of the elements. 



This subject will be more particularly enlarged on here- 

 after, when the joints of sedimentary rocks come under con- 

 sideration. But it may be proper to offer now a brief sketch 

 of the structure of igneous rocks, in order to compare it with 

 that of other formations, and to test the opinion which I 

 have elsewhere advanced, M that the structure of rocks is not 

 dependent on any one particular mode of formation, for each 

 individually, whether igneous or aqueous, by placing the mo- 

 bile particles under circumstances favourable to the exertion 

 of cohesion, effects the same object by different means." 



When igneous rocks occur in large masses, they are found 

 to be traversed by systems of parallel joints or fissures, by 

 which the whole mass is divided into blocks, varying in size 

 and figure according to the nature of the rock. Sometimes 

 the forms of the resulting blocks are very irregular; but at 

 other times they possess such a surprising symmetry of di- 

 mensions that they deserve to be termed crystals. A closer 

 inspection shows that the minerals of which these blocks are 

 composed, are frequently so arranged as to produce coarse, 

 fine-grained, and other varieties of rocks. These varieties 

 either occur together, side by side, in layers both parallel and 

 regular, and even unequal in size, and variously convoluted ; 

 or they are disposed in nodular and spheroidal concretions ; 

 or they assume the appearance of veins. All these arrange- 

 ments vary much in their dimensions and directions : some- 

 times they are so small as to be confined to individual blocks; 

 but more commonly they extend through several adjacent 

 blocks ; and not unfrequently they prevail over considerable 

 masses. In the last case, the layers and veins will be found 

 to affect positions parallel to the principal joints or fissures. 

 Under the same head must be placed the lamellar and slaty 

 structure, as also resulting from a peculiar arrangement of the 



