1858.] HARKNESS JOrNTINGS. 99 



secting the S.W. of Ireland, and it is equally inapplicable to the 

 Devonian sandstones and the Carboniferous limestones of the area 

 under consideration. 



Mechanical force, operating in the form of pressure, seems to have 

 been an agent more capable of producing these parallel planes. And, 

 when we consider that this pressure has produced important changes 

 on the particles of rocks which have been subjected to it in the 

 south of Ireland, we may form some idea of the position in which 

 rocky masses subjected to this force were placed. One of the pro- 

 ceeds of this pressure has been cleavage, the strike of the planes of 

 which is nearly E. and W., and these planes have been elongated in 

 the direction of their dip. Any mass of rock, the mineral nature of 

 which was unfavourable to the production of cleavage-structure, if 

 enclosed between two other masses, capable, from their lithological 

 nature, of assuming a cleavage-structiire, would be subject to com- 

 pression, from the re-arrangement of the particles of the enclosing 

 rocks undergoing this form of structure. The mass enclosed, so 

 operated on, would have a disposition to extend itself at right angles 

 to the planes of pressure ; but, being rigid, the result would be the 

 breakiiig up of this mass into a series of parallel joints, such as are 

 exhibited by the N. and S. joints in the Devonian and the Carboni- 

 ferous strata of this locality. The mode in which the fossils of the 

 limestone are distorted, previously alluded to (p. 96), is a strong 

 confirmation of the supposition that pressure exei-ted an important 

 influence in the production of the N. and S. divisional jointing 

 planes. 



The effect of pressure, in connexion with rocks of the Carboniferous 

 age, is stiU further manifested among the coal-measures of Kanturk 

 in this county. Of these Mr. Jukes remarks : — " These coal-measures 

 are commonly highly inclined and contorted, and often inverted ; and 

 the coals are not only changed into anthracite, but squeezed and 

 crushed so as to be only got in small dice-like fragments. The 

 regularity of the beds is also interfered with, so that the beds of 

 which the original thickness was probably a couple of feet or so, 

 have now for many yards only one or two inches, and then suddenly 

 expand into large pockets of coal, twenty or thirty feet in thickness. 

 Coal-mining here is conducted like vein-mining*." This is not the 

 only change which the coal here has undergone. 



It often presents an external form of a prismatic character, in 

 which the bundles of the prisms are arranged at right angles to the 

 laminae of deposition ; an aspect, which, if the coal possessed a semi- 

 metaUic lustre, would give it an appearance far more nearly re- 

 sembling manganese than coal. 



The whole strata in the neighbourhood of Cork have been subjected 

 to violent forces ; they exhibit contortions and flexures to a very 

 ^eat degree ; and the pressure which must have arisen in consequence 

 of these great disturbances, seems to have produced not only cleavage, 

 but also a great amount of jointing. It is a matter of some interest 



* Jukes's Manual of Geology, p. 445. 



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