ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxvii 



divisional planes termed joints. Examples of rocks cnt by two 

 cleavage planes and two planes of joints, besides the planes of the 

 original bedding, are occasionally seen, so that the most complicated 

 appearances are produced, particularly when the cleavage is neither 

 parallel nor at right angles to the planes of deposit. Cleavage may be 

 sometimes seen to have produced ridges of hills, during denudation, in 

 a direction different from the strike of their beds, even diagonally to 

 that strike, as, for example, the range in Ireland, known from one of 

 the hills as the Chair of Kildare. We there see a variety of sub- 

 stances, slates, sandstones and trappean rocks, even a hard and beau- 

 tiful porphyry among the rest, all cut by a general line of cleavage, 

 ranging diagonally to the strike of the beds. Again, we, in some 

 regions, find beds contorted in all directions, as well horizontally as 

 vertically, cut by some general line of cleavage, one common to some 

 great district, and of w^hich these contortions constitute only a minor 

 portion. 



To bring the subject within grasp, great districts have to be care- 

 fully worked out for their cleavage planes, not neglecting any of their 

 modifications. The conditions of the rocks themselves, their varied 

 compositions, flexures, and probable state as regards cohesion at the 

 time of one or more compressions from the exertion of mechanical 

 force, have to be carefully weighed. Neither should those divisional 

 planes, the joints, be neglected, cutting, as they often do, through a 

 variety of rocks, and yet no trace of the least shift of their sides 

 observable. We are indebted to Professor Sedgwick for general views 

 as to the direction of cleavage in parts of our island, and Mr. Sharpe 

 has brought the movement of the particles of rocks in planes of 

 cleavage, also noticed by Professor John Phillips, prominently be- 

 fore you, and at the same time has directed your attention to the 

 general character of cleavage planes in certain extended districts. 

 Let us hope that those who have taken part in this important inves- 

 tigation, whatever their views of the cause of cleavage may be, wull 

 continue their labours in this field, and that we may have the results 

 communicated to this Society for that honest discussion which usually 

 follows the reading of papers in this room. 



In his communication on the comparison of the structural features 

 of the Appalachians with those of certain disturbed regions in Europe, 

 Professor Henry Rogers considers cleavage, and points out that the 

 alterations of internal structure and texture of the rocks in the Appa- 

 lachian region prevail much further to the north-west than the limits 

 of the igneous rocks. These alterations are noticed as an induration 

 of all the rocks, the crystallization of the limestones, the debitumeni- 

 zation of the coals, and an extensive cleavage of the argillaceous 

 masses. The planes of cleavage are remarked as dipping almost in- 

 variably with the closely-folded beds towards the south-east, and it is 

 stated that the cleavage is approximately parallel to the axes-planes, 

 a position inferred to be in accordance with a law applicable to all 

 plicated districts. According to the Professor, the cleavage dip of the 

 Alps is in consequence directed inwards from both sides along the 

 great axes-planes, so that the arrangement of the plications and the 



