109 



Cleavage. Faults. 



7 46' North of East. 

 33 31' North of East. 34 22' North of East. 

 30 30' South of East. 

 10 South of East. 



The cleavage planes are distinguished from the joint planes by a 

 peculiar flaggy or platy structure developed in the rock-mass, parallel 

 to their direction. This structure the author thinks to be the result 

 of pressure ; and that it indicates that the cleavage planes are per- 

 pendicular to the lines of maximum force ; he considers the cleavage 

 planes to have been developed while the rock was yet soft. The 

 joint planes, on the contrary, which are conjugate to the cleavage 

 planes, are considered as perpendicular to the lines of minimum 

 force of compression ; they were formed by the shrinking of the rock 

 mass, were subsequent to the cleavage planes, and formed when the 

 rock was hard. 



Having established the geometrical relations of the structural planes 

 of the conglomerate, the author then deduces from them the mecha- 

 nical forces which have been at work in bringing the district to its 

 present condition and form. He believes that the method he has 

 adopted in reference to the conglomerate of the county of Waterford 

 is applicable to the physical structure of other districts ; and that 

 his results, if confirmed by corresponding results in other districts, 

 of which he is confident, will prove to be a substantial addition to 

 the arguments in favour of the mechanical theory of slaty cleavage. 



The paper was accompanied by four diagrams illustrative of the 

 cleavage and joint planes of Portally Head, Swiny Head, Shanooan 

 Head, and of the reversed fault at Portnashrughann. 



