CoLK — The Picture- Rock r>r FScrihed Rock near Ralliviiilhtn. 117 



hamnicr. This plaiic rormed one siilu nr uihvx uf Uie oi'igiiial joiiiL-ci'ack, 

 and Llie crack itself has been filled by cryHtallinc^ material. During the 

 infilling of the cracks, chemical changes must have taken place in the rock to 

 varying distances on cither side. This action may be judged, from field- 

 inspection, to be hydrothermal, and to have gone on when the mass lay 

 buried deeply underground. Sucii infiltrations, however, whereby original 

 planes of weakness become strengthened, may take place even in calcareous 

 shales. Several interesting examples were dug up some ten years ago near 

 Harold's Cross, Dublin. The shale had crumbled away, except where 

 cemented by calcium carbonate on either side of the joints, which formed 

 two series crossing approximately at right angles. The layers of rock thus 

 presented an open lattice-structure or meshwork of remarkable regularity, 

 and detached pieces formed perfect crosses of stone. 



On examination in the laboratory, the igneous sheet of the Picture-Eock 

 proves to be a fine-grained dolerite, almost andesitic on its surface, where it 

 originally contained some glassy matter. 



It has been subjected to extensive alteration. In its present condition, 

 rich in chlorite, it is a typical diabase, in Hausmann's sense of the term.' 

 The specific gravity of two spheroids is 3"05 and 3'07 respectively, giving an 

 average of 3'06. That of one of the strengthened layers along the joint- 

 surfaces is 2'91. This difference in density is not one on which stress can be 

 laid, as a greater degree of hydration probably now prevails among the 

 minerals near the jomt-surfaces than among those near the centre of the 

 spheroids. When the rock is broken along a joint-surface, abundant limo- 

 nite is seen. Pyrite is a common constituent of the infilling of the joints, 

 and has been introduced freely in specks into the rock on either side. 



In microscopic section, the veins now occupying the joints are seen to 

 consist of fibrous green amphibole (hornblende or actinolite) granular quartz, 

 pyrite altering into limonite, and occasional small rounded granules with 

 high refractive index, which are very probably sphene. The aspect is that of 

 a mineral vein on a small scale; and it is clear that the agents which l)rought 

 up the infilling materials exerted considerable infiuence upon the bounding 

 walls. I have elsewhere^ refeiTcd to the mineralizing eflect of a granite 

 magma on its surroundings, and to the production of considerable crystals of 

 amphibole. The hydrothermal action that led to the filling of the narrow 

 veins tliroughout tlio Picture-Kock seems, however, to have actually imported 



' Rosenbusch has unfortunately appropriated the term 'diabase' for unaltered pyro.xene-plagioclase 

 r..,ks. 



- " On tlie growth of crystals in ihe contact-zone nf Granite and Ampbibolite," I'roc. lioy. Irisli 

 Acad., vol. XXV., sect. B (1905), p. 117. 



