222 Notices of Memoirs — On ^' Jointing ^^ and Slaty Cleavage, 



and furtlier elucidated by Tyndall and Haughton, necessitates a 

 strictly coincident strike of beds and cleavage, — and after pointing 

 out that in North Devon, Charnwood Forest, the Lake District, Ire- 

 land, and elsewhere, differences have been found to exist between 

 the two strikes, Dr. King remarks : '' The hypothesis proposed by 

 m^^self does not limit the action in force to the direction just stated 

 ^^perpendicularly to the resultant planes] : it admits of the force being 

 applied in various directions against pre-existing divisional planes : 

 therefore, cases answering to those noticed in the last paragraph do 

 not stand out as antagonistic to it, but as coming within the category 

 of its resultant phenomena. Obviously a force acting in a line deviat- 

 ing to some extent from perpendicularity to joint-planes will bring 

 them into proximity, and weld cognate planes together; though,, 

 perhaps, not so effectively as if it had been exerted perpendicularly 

 to them. Moreover, as noticed at p. 638 [limestone near Cork, with 

 complex jointings], jointings of both sets forming the meridional 

 system, one striking east of north, and the other west of north, may 

 occur separately in a great thickness of superimposed rocks, repre- 

 senting two different geological periods — say Devonian and Silurian ; 

 therefore pressure exerted uniformly and simultaneously throughout 

 the mass, while acting perpendicularly to the strike of the east-of- 

 north joint-planes in the Silurian rocks, would also be acting obliquely 

 to the strike of the west-of-north jointings in the Devonians : yet it 

 may be consistently held that in both instances slaty cleavage, though 

 necessarily of two discordant strikes, could result from one and the 

 same movement. It appears to me that the above points not only 

 elucidate cases of non-coincidence between the strikes of cleavage and 

 bedding, but they afford a fair and full explanation as to why it is 

 that ' cleavage is symmetrically related to axes of movement of the 

 strata' [Phillips] . So much cannot be said of the purely mechanical 

 hypothesis." (pages 651-2.) 



The author states his view of the nature of jointing as follows : 

 "Fully believing that jointing is in the main a physical pheno- 

 menon, the particular hypothesis which most strongly suggests itself 

 to my mind, to account for it, is, that it is the product of divisional 

 forces akin to those which give rise to mineral cleavage. The 

 difference between the two phenomena arises, perhaps, from the 

 forces, in the case of jointing, being subordinated to terrestrial 

 magnetism — to laws pervading the crust of the earth ; while those 

 productive of mineral cleavage are obedient to crystalline polarity — to 

 laws limited to specific mineral substances or solids. Or, to disarm 

 adverse criticism, usually evoked when anything is ascribed to the 

 mysterious agents that have been introduced (though I wish it to be 

 understood that the circumstances indicate what my leaning is), it 

 may be safer, expressing myself as I did in 1857, to regard both 

 divisional structures as the result of different manifestations of one 

 and the same force." (p. 632.) 



The " electro-chemical agents " recognized by the late Evan 

 Hopkins as always silently at work " dissolving, recomposing, and 

 cleaving " both the deep-seated and the superficial strata of the 



