446 Dr. Boase's Inquiry into the Nature 



" may be carried on indefinitely, or at least as far as the opera- 

 tion is not interrupted by a mere mechanical difficulty." Again, 

 " should any one assert that the subdivision of slate-rocks into 

 rhombohedral solidslmplies three planes of cleavage, we might 

 reply, that such solids are not capable of indefinite subdivision 

 into similar solids, except in one direction, viz. that of true 

 cleavage; and in this way we may generally distinguish the true 

 cleavage planes from the joints." Now we are also informed (at 

 p. 477.) that"the cleavage planes passalike through all thestrata, 

 the coarse beds and the finer, the twisted and the straight." 

 We might be led from this to conclude that all these strata are 

 susceptible of indefinite subdivision; but such a mistake would 

 be corrected in the page following, which states that " where 

 the slaty cleavage is very perfectly brought out, the structure 

 of the rocks always makes an approach to homogeneity ; 

 where the quartzose beds of coarse greywacke abound very 

 much, the cleavage is seldom very perfect, or is at least chiefly 

 confined to particular strata; and where the coarse beds pre- 

 dominate, the slaty structure almost entirely disappears." 

 This is contradictory to the preceding statements, for it shows 

 that cleavage planes do not pass alike through all the strata, 

 that they are not indefinite, and that they are often as far 

 apart as the rhombohedral joints, so that they cannot be 

 thus distinguished from the latter kind of structure. In 

 drawing, therefore, the sections to illustrate this subject, all the 

 lines in the slate should not be extended through the coarse 

 rock, but only one here and there should have been made to 

 intersect all the strata. 



It appears, then, that the proposed restriction of " old 

 terms" is not well adapted for practical purposes ; and for this 

 simple reason, because the definitions of these terms require a 

 preliminary knowledge of the direction of strata; a proposi- 

 tion which cannot always be solved, and even when ascer- 

 tained to the satisfaction of one party, may be disputed by 

 another as purely hypothetical. 



We therefore contend that the slaty cleavage of rocks has 

 not necessarily any connexion with the direction of beds or 

 strata which have been formed by, and mark the order of, 

 deposition, but that it is, as elsewhere remarked, " a pecu- 

 liar kind of structure common to all rocks, both igneous and 

 aqueous, and which has most probably resulted from a cry- 

 stalline arrangement of their particles during the process of 

 consolidation." The Professor is also of opinion that the re- 

 gular cleavage is to be attributed to crystalline action; but I 

 am not quite sure whether he supposes it to have operated on 

 solid rocks, as seems to be implied by the observation (at p. 

 477.) that " crystalline forces have rearranged whole mountain 



