144 The Rev. Maxwell H. Close, 



It is, of course, quite possible that there may be friction slicken- 

 sides on planes of dislocation in the granite. There must have 

 been such planes produced during the disturbances after the Car- 

 boniferous age, when the granite had been thoroughly solidified, 

 and great friction must have taken place along them ; but the 

 slickensides so frequently to be seen in the granite joints of this 

 neighbourhood are, at least as a general rule, most clearly struc- 

 tural and not the result of friction. The slickenside striations of 

 the quartz coating of a joint surface are often accompanied by ca- 

 pillary schorl, the needles or fibres of which are accurately 

 parallel to the slickenside striation and unquestionably form part 

 of the phenomenon. The great majority of the slickenside-bear- 

 ing joints of a neighbourhood have a very observable nearness of 

 direction with each other ; the mean dip of these surfaces is at 

 about 30°, so that the variation in the direction of the planes is 

 only one half that of the strikes of the planes. Moreover, the 

 striations are not only strictly parallel on the same surface, but 

 they very seldom deviate much from the mean direction in their 

 vicinity ; showing that their directions have been influenced by 

 some common cause. This can be well seen in many places ; of 

 which one of the most easily indicated and accessible is the shore 

 at Sandycove (between Kingstown and Dalkey), from the bottom 

 of Burdett Avenue for some distance eastwards. 



The granite is often penetrated by dykes and veins of eurite 

 (which may be called a fine close-grained granite with very little 

 or no mica) and by veins of quartz. These, when they intersect 

 can often be seen to be of different ages; and they are sometimes 

 faulted. The eurite veins may be intrusions of later date than the 

 solidification of the surrounding mass, or they may be only infil- 

 trations into contraction fissures ; the quartz veins have been 

 doubtless formed by infiltration. 



Occasionally, though apparently very rarely, the granite in 

 this neighbourhood exhibits an obscure concretionary structure. 

 Sometimes a freshly exposed joint surface will show indistinct 

 concentric rings of colour two or three feet in diameter, usually 

 iron staining, which might, at first sight, lead to the supposition 

 that the joint had cut across a concretionary spheroid ; but it 

 will usually be manifest on closer inspection that the phenomenon 

 is confined to the joint itself. In some places, as near Mur-. 



