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LXX. — NOTE ON THE OCGUEEENCE OF GEANITE POE- 

 PHYEY IN THE THEEE-EOCK MOUNTAIN, COUNTY 

 DUBLIN. By PEOFESSOE J. P. O'EEILLY. 



[Eead, June 15, 1885.] 



The mass of granite which has received the name of the Three-Rock 

 Mountain, though apparently of very simple outline, presents in cer- 

 tain places channels and gorges which become recognizable from a 

 distance towards sunset when the rays of the sun, passing at a very 

 low angle, bring out the inequalities of the surface very markedly. 

 It may thus be seen that certain shallow gorges and channels occur, 

 and that their directions are apparently parallel. One of these is 

 known to the quarrymen of the village of Sandyford and environs 

 as " Phuldhu," and in the southern end of it I found last year a 

 block of granite, partly detached from the mountain, the texture 

 of which struck me as interesting by reason of the great develop- 

 ment of the orthose crystals. I had the blocks separated from the 

 main mass, and saw-cut and polished in the part where these 

 crystals appear. The slab resulting, now exhibited, presents a 

 very agreeable colour, owing to the size and structure of the 

 flesh-coloured orthose crystals. I had already noticed the direc- 

 tions of these gorges, and included this one in the group of joint- 

 ings, having the mean direction N. 6° 57' W. I, therefore, assume 

 that the porphyritic character points to the presence of one of these 

 joints in the gorge, along which the same character may be found 

 to present itself in other places. Were the rock thus character- 

 ized more abundant, it might give rise to an adaptation for orna- 

 mental purposes; but I am not aware of it occurring with this 

 appearance in any other locality of the mountain. 



