98 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9, 



sides of the open joints ; as these joints would have afforded facilities 

 for extension rather than the planes of stratification. 



Cause of the Jointings. — With regard to the source to which 

 jointing is to be attributed, this is a matter which is, as yet, in- 

 volved in considerable obscurity. 



Prof. J. Phillips has described the series of joints which are so 

 well developed among the sedimentary rocks of Yorkshire, and also 

 pointed out the effect of a difference in the mineral composition of 

 the rock on these joints*, but without, however, assigning a cause 

 for the occurrence of these structures f. Sir Roderick Murchison has 

 also noticed the results of varying lithological composition on 

 jointings, and has also observed, that among the Silurian rocks, joints 

 alter their direction with every change in the axis of elevation. 

 He still further remarks that rocks ^^have a jointed structure, the 

 result of crystalline action, and that the divergent directions in which 

 the joints are arranged is the consequence of the mechanical process of 

 elevation J . " On the subj ect of j oints. Prof. Sedgwick § observes, that 

 while cleavage is the result of crystalline action, joints have their 

 origin in mechanical causes. The latest observations which we 

 have on the phenomena of jointing, and the forces which have given 

 rise to them, are to be found in Mr. Jukes's ' Manual of Geology,' 

 1857, and as his observations have to a great extent been derived 

 from the south of Ireland, they have an important bearing on the 

 district under review. Mr. Jukes is to some extent disposed to 

 attribute joints to the " natural result of the shrinkage or contraction 

 of rocks upon consolidation ||;" but, when aUuding to the extensive 

 joints which make their appearance on the north side of Bantry Bay, 

 and to others of a like character, he remarks, that *' such unlimited 

 joints were very probably produced, not from any internal shrinkage 

 on the mere consolidation of the beds, but from a simultaneous 

 yielding of the whole mass to a great expansion or stretching 

 force ^." These theories resolve themselves into the consideration 

 of two forces, — the one crystalline, and the other mechanical. This 

 latter, however, presents itself to us in two aspects, — the one acting 

 by compression, and the other operating in an almost opposite 

 manner, — ^namely, by simple shrinkage from drying and consolidation. 

 What has been said with reference to the cutting of the quartz- 

 pebbles by joints in the Devonian conglomerate of the south of 

 Ireland has a hostile bearing to the theory of consolidation in con- 

 nexion with these divisional planes; and there are other circumstances 

 which are antagonistic also to this hypothesis. Mr. Jukes himself 

 finds that it is not calculated to account for the huge joints inter- 



* ' Mountain-limestone of Yorkshire,' p. 94. 



t Prof. Phillips presented, in 1834, " Notices" in reply to a question on this 

 subject, proposed by the British Association in 1833. Eeport Brit. Assoc. 1834, 

 Sect. p. 654. See also Hopkins, Eep. Brit. Assoc. 1838, Sect. p. 78. 



I Silurian System, pp. 246, 247. 



§ Synop. British Palseoz. Eocks, &c., p. 36. 



!i Manual, p. 193. ^ Op. cit p. 194. 



