448 Dr. Boase's Inquiry into the Nature 



less regularity by parallel systems of fissures or joints," but 

 that " the jointed structure is best seen in unstratitied rocks, 

 such as basalt and certain varieties of granite." The joints 

 are certainly very distinctly marked in these rocks, but not 

 more so than in very many stratified rocks ; for example, 

 in all the older slates both primary and fossil iferous; and with- 

 out doubt the jointed structure is much more perfectly deve- 

 loped in the sandstones of many parts of Scotland than in the 

 masses of trap with which they are associated. The slaty 

 structure however, when present, is so conspicuously promi- 

 nent, that it obscures, or rather diverts the attention from, the 

 joints by which the rock is divided into crystal-like concre- 

 tions. But the jointed structure will, I think, be found to 

 occur more or less distinctly in all rocks that are completely 

 solidified. It is sufficient however, for the purpose of the 

 present inquiry, that stratified rocks have sometimes the 

 jointed structure ; in the slate of Cumberland and Wales, for 

 instance, " besides the planes of cleavage, we may often find 

 one or more sets of cross joints, which, combined with the 

 cleavage, divide the rock into rhombohedral solids." 



It is worthy of remark that the regular concretional forms 

 of slate rocks are, almost without exception, varieties of the 

 rhomboid ; a figure which is also frequent in porphyry, trap, 

 granite, and other igneous rocks; and it must also be ob- 

 served that these rhomboids, especially in compact rocks, are 

 likewise jointed diagonally, a fact which may be detected on 

 the face of the weathered rock, or, if much decomposed, by its 

 falling into three-cornered pieces under the blows of the ham- 

 mer; a fact which is important as accounting for the position 

 of some veins which are disposed diagonally to the principal 

 systems of regular and cross veins. 



How are joints to be distinguished from cleavage or slate 

 planes ? The Professor says that " the joints are at definite 

 distances from each other, and a mass of the rock between 

 them has, generally speaking, no tendency to cleave in a di- 

 rection parallel to them." It is admitted immediately after 

 that there are exceptions to this rule, for " a slaty and jointed 

 structure are often exhibited together; in such a case the 

 rock has fissures or joints and true cleavage planes coincident 

 with each other ; and again, when a cleavage is imperfect, it 

 is sometimes only exhibited by parallel planes at definite di- 

 stances, in which case it may be difficult to say whether the 

 phaenomena are to be classed with joints or cleavages." 



This difficulty is said to have arisen from one set of joints 

 having been formed mechanically on the planes of cleavage. 

 If this be the case, we would ask how it could be determined 

 that the cleavage passes alike through all the strata in Wales, 



