142 



The Rev. Maxwell H. Close, 



tioned again— the mica slate can be seen in close proximity to 

 the granite; crystals of chiastolite are to be found in it there 

 also. The contact is seen in some places on the west side also 

 of the granite, but only imperfectly, and not under specially in- 

 teresting circumstances. 



It has been suggested that some of the granite of the main 

 mass may have been produced by extreme metamorphism from 

 the rocks in which it is contained; but the similarity of its com- 

 position in different places, as far as is known, seems to throw a 

 very great difficulty in the way of this hypothesis. No doubt, as 

 the granite has acted upon the Silurian rocks with which it has 

 come in contact, these rocks must, in some way or other, have 

 reacted upon the still tractable materials of the un solidified 

 granite. Probably it is to such reaction that the interesting 

 phenomenon to be seen on the S. slope of Rochestown Hill (a 

 locality already mentioned) is due. In a quarry there, just N. 

 of the garden wall of Killiney Park, it may be observed that the 

 crystals of black mica in the granite are arranged in layers par- 

 allel to the bounding surface between the granite and the slates. 

 This is visible for a distance of ten or twelve feet from the boun- 

 dary, which is very abrupt and definite. 



The chemical and mineral composition of this granite has been 

 elaborately investigated by the Rev. Dr. Haughton, f.r.s. For 

 the full results of his analyses and the discussion thereon, the 

 reader is referred to Dr. Haughton's Paper in the Quart. Journ. 

 Oeol. Soc, London, vol. xii., 1856; and to the joint Paper, by 

 Professor Jukes and himself, in the Trans. Royal Irish Aca- 

 demy, vol. xxiii., 1858. 



The analyses show comparatively slight differences in the pro- 

 portions of the constituents of the rock in different localities 

 The following table gives the mean chemical composition of 

 specimens obtained from eleven generally widely separated 

 places : — k 



oiuca, . , 

 Alumina, 





14-81 



Peroxide of Iron, 



• • 



► 2-22 



Lime, 



* » i 



1-63 



Magnesia, . 



• • 



0.33 



Potash, . 



• • i 



5-11 



Soda, . . ► i 



• • 



2-79 



Loss by ignition, . 



► • # i 



, 109 



10005 



