108 ROCK-METAMORPHISM. 



of illustrating a phenomenon abundantly characteristic of me- 

 chanically deposited rocks. 



I do not deny that pressure, under certain conditions, has 

 produced parallel fractures in rocks; and examples could be 

 cited of a mechanically developed parallel divisional structure 

 such as some geologists have taken for jointing. Still, as 

 jointing is common in its indisputably typical or normal condi- 

 tion, without a tittle of evidence of mechanical pressure having 

 been in any way concerned in its development, obviously a 

 physical causation must be ascribed for the phenomenon. As 

 stated in my Report, there are miles and miles of Carboniferous 

 limestone, in nearly horizontal beds, forming the bare surface 

 of many parts of counties Clare and Galway, in which jointing 

 is wonderfully developed ; but nowhere is it accompanied by 

 evidences of crush or stratal disturbance involving a mechanical 

 causation. On the contrary, the phenomenon so closely simu- 

 lates mineral cleavage in fineness, and it is so completely 

 divested of all indications of supervened compression, that the 

 view of its mechanical origin, advocated by Daubree and others, 

 must be regarded as completely at fault. 



M. Daubree, however, by way of adducing some evidence of 

 the required pressure in rocks, has brought forward a case 

 notified by Harkness and examined by myself*. It is an 

 instance in Carboniferous limestone near Cork, which not only 

 comprises typical jointing, but shows indisputable evidence of 

 the rock having undergone powerful compression. This case, 

 however, is altogether valueless with reference to the mechanical 

 hypothesis; for it can readily be proved that the jointing 

 (which is meridional) has been developed after the rock had 

 become compressed. Clearly the divisional structure in this 

 instance is altogether independent of compression. The fact is, 

 the Cork limestone (as well as its associated rocks) has been 

 flexured into east-and-west rolls, corresponding in direction with 

 an old equatorial jointing, traces of which are still to be seen in 

 the roughened and dislocated divisional planes everywhere 

 present. On the contrary, the jointing referred to by Daubree 

 cuts cleanly through the disturbed rocks, north and south, and 

 holds on for miles in the same way as the meridional jointing 

 of Ireland generally. 



* Op. cit, pp, 638 & 639, 



