of the Structure of Rocks. 449 



whether coarse or fine beds? For if in the coarse rocks the 

 cleavage planes are far apart like joints, how can it be known 

 that the slaty cleavage ever passed through the coarse beds ? 

 It may be after all only the joint or supposed mechanical 

 fissure, which in the slate is coincident with some of its lines 

 of cleavage. We are therefore inclined to take a much more 

 simple view of the subject, viz. that the slaty cleavage and 

 joints have been formed by the same cause — that they are 

 both crystalline structures ; — not denying, however, that ex- 

 ternal force may have operated in the direction of any of these 

 crystalline planes, but maintaining that this coincidence has 

 no connexion with the production of the jointed structure. 



The Professor seems to think, because the cleavage can be 

 carried to a greater extent on the slaty laminae than on either 

 of the other planes of structure, that there is no cleavage 

 parallel to the latter. It has already been shown that a true 

 cleavage is allowed to be sometimes at definite distances from 

 each other ; therefore the smallness of the space between two 

 parallel surfaces, that is, the thickness of the layer of rock, 

 does not essentially constitute a plane of crystallization, or a 

 perfect cleavage. Now, a large rhombohedral mass of slate 

 may be divided, in the direction of its laminae, into several 

 thick plates, which can be again broken across so as to afford 

 numerous small rhomboids. These are always developed at 

 the surface of such a rock by decomposition; but can also be 

 disclosed in the perfect unchanged state by mechanical divi- 

 sion: and even thin roofing-slates, exhibiting no trace of 

 joints, and therefore no cracks and fissures, may be broken 

 across, in certain directions, so as to give lateral planes in- 

 clined to the surface of the slate at considerable angles. 

 Moreover, do not the crystals of some simple minerals afford 

 a perfect parallel ; crystals of mica, for example, being foli- 

 ated in the direction of one cleavage plane, and not in the 

 other; and in such cases would any one maintain that the 

 foliated planes are crystalline, and that the others have arisen 

 from mechanical violence? 



This notion of a monogenous cleavage in stratified rocks 

 has been extended by the Professor to some instances of ig- 

 neous rocks ; by which, indeed, he has avoided one difficulty, 

 but augmented his labours by bringing into existence a seven- 

 fold increase of similar exceptions. 



The cleavage of granitic rocks, or grain, as the Professor 

 terms it, is indicated by parallel laminations, which are al- 

 lowed to have arisen from crystalline action; the cause, then, 

 of this and of the slaty structure or transverse cleavage is 

 identical. In the granite ofCarclaze, near St. Austle, specially 

 quoted to illustrate this opinion concerning the grain of ig- 



Third Series. Vol. 7. No. 42. Dec. 1835. 3 M 



