On " Jointing " and Slaty Cleavage. 221 



■^'equatorial," bearing; tlie intermediate bearings, wMch seem to have 

 wide ranges, being termed " E. and W. meridionals," and " N. and S. 

 equatorials." The meridional joints appear to be the most frequent 

 and important. The cause of the jointing is not known: little is 

 allowed to contraction and tension ; and the less definite agencies of 

 chemistry, electricity, and magnetism are referred to as probably 

 concerned in its production. 



It is also implied (p. 642) that the known secular variation of 

 the magnetic meridian may have existed in very early times, and 

 that " the magnetic polar currents," thus changing their direction, 

 may have produced more or less divergent meridional jointings in 

 course of ages. The author is "not acquainted with irwe jointing in 

 rocks of later periods than the Jurassic in the British Isles " ; but 

 accepts the divisional structure of the Gypsum at Montmartre, and 

 of the Nagelfluhe of the Ehigi, as probably Tertiary jointing. 



The partial occurrence of such vertical jointing in one set of 

 beds, over- and under-lain by jointed strata, is not only pointed out 

 as a natural phenomenon, but the fact of such a set of joints having 

 been laterally shifted into an oblique position (like a set of sloping 

 books between two parallel shelves) by lateral thrusts, and here 

 termed ''^dislocated jointing," is clearly expounded as a real cause 

 of what is sometimes regarded as a local cleavage limited to one 

 stratum or stratal series. It is shown also that jointings may 

 really have determined the direction of dislocations, although the 

 uppermost (latest and visible) set of joints may not coincide with 

 the faults ; for the lower strata may be cut up by older, deep- 

 seated, and more widely pervading joints, giving direction to the 

 ■ main movements (p. 638). 



"■ We have had," says the author, " examples before us of jointing 

 both unlimited (as in the papery variety), and intermittent. Mineral 

 cleavage I hold to be a superinduced divisional structure, its planes 

 having no relation to incrementation or depositional growth : so it 

 is also with jointing" (p. 640). He admits, however, that slaty 

 cleavage in some rocks may be the result of pressure, rearranging 

 and parallelizing the constituent particles and contained cavities, to 

 produce planes of weakness ; but unless the cleavage be practically 

 unlimited or papery, — that is, if it be intermittent with intervals 

 of even less than an inch, he prefers to regard the divisional planes 

 as having been due to the jointing, whether in original vertical 

 position, or modified by disturbance of the stratal mass, with or with- 

 out lateral pressure. The welding together of joint faces is mentioned 

 as a frequent condition, accounting for what is sometimes described 

 as imperfect cleavage ; and what has been treated of by others as 

 secondary cleavage, Prof. King thinks may be also accounted for by 

 modifications of joint- structure under pressure. The difSculty of 

 explaining the discrepancy between the strike of bedding and the 

 strike of cleavage in several known cases is met by the supjDOsition 

 of the lateral crush having been subsequent and oblique to one or 

 more sets of original jointings. Thus, after stating that the mechanical 

 theory of cleavage, as advanced by Sharpe, determined by Sorby, 



