478 jET. B. Medlicott — On Faults in Strata. 



is taken from the patch of Old Eed Sandstone on the coast of the 

 county of Waterford. The rock covers an oblong area of about 

 twenty-four square miles. Its most conspicuous variety is a massive 

 conglomerate, some 200 feet thick, to which, for the sake of greater 

 uniformity, all the observations were confined. Eeadings were taken 

 of the bearings of 345 well-marked planes of fracture, whether joints 

 or cleavage, faulted or unfaulted. When those readings are plotted, 

 as in the accompanying diagram, no case being omitted, the great 

 majority of them exhibit a marked natural arrangement, not only in 

 single groups or systems, but these again showing a distinct distribu- 

 tion in pairs, in which the mean directions of the two groups are at 

 right-angles, or conjugate to each other ; forming together what Dr. 

 Haughton calls a conjugate system. Two such double systems are 

 conspicuously prominent, A C, and A' Q' : the A groups lie on 

 either side of north-east ; and the C groups on either side of north- 

 west. Traces of two other conjugate systems. A!' G", A!" C", are 

 discernible. There is, of course, an arbitrary margin in the demar- 

 cation of these groups : A and A^ have each a range of 26°, while 

 for K'" there is onlya space of 12° left. It may be admissible that the 

 range of effects of any force should be proportionate to the intensity 

 of the action ; but this variability would be a grave element of uncer- 

 tainty when the systems were less marked than in the case before us. 



The planes of cleavage, of jointing, and of faulting, are separately 

 noticed. A platey or flaggy structure, varying from a quarter to 

 four inches in thickness, was observed, having the direction of the 

 group A, sometimes of A' : "now as the lines of greater force in a 

 system of elevated beds must lie in the vertical plane, or nearly so, 

 which contains the direction of the dip of the beds, and as the strike 

 of the beds is intermediate between the systems A and A'^, it is 

 evident we have a close approximation to Sharpe's law," that cleavage 

 planes are at right-angles to the direction of the greatest force. The 

 aforesaid platey structure, therefore, represents cleavage, as formed 

 in this conglomerate ; and thus a key was obtained " to the explana- 

 tion of the physical structure of the whole district," the direction of 

 the main forces being ascertained. Dr. Haughton considers a more 

 or less pliant condition necessary to the production of cleavage ; and 

 that the force which produced it was probably the first that acted 

 upon these rocks. 



The true joint -planes are at right angles to those of cleavage ; 

 and it is a characteristic of both that the hard pebbles of the coki- 

 glomerate are cut clean across by them. In the case of the cleavage 

 planes this is explained by the development of a latent cleavage 

 even in the hard pebbles. The jointing is supposed by Dr. Haugh- 

 ton to have been produced by the shrinking of the consolidated rock- 

 mass, consequent upon evaporation and drying, the planes being 

 determined at right-angles to the lines of minimum force to which 

 the rock had been subjected, and t'herefore conjugate to the cleavage 

 planes. 



There are other planes of fracture besides those of cleavage and 

 of jointing. Of twenty cases of faulting, nineteen occurred in the 



