164 



Proceedings of the Royal Irish Acadennj. 



XXII. — Eepoet on the Miceoscopicai- Stkuctitre of Kocks. JSTo. 3. 

 By G. H. KiNAHAN, M. E. I. A., &c., &c. 



[Read February 8, 1875.] 



Carnsore Granite, Co. Wexford. 



The granite from which the specimen was procured occupies a 

 small tract on the S. E. shore of Wexford, in the neighbourhood of 

 Carnsore Point. The mass of the rock is evidently of metamorphic 

 origin, but in the vicinity of the Point there appears to be an intrusive 

 mass of slightly different gi-anite. The typical rock is of a reddish 

 greyish colour, containing large crystals, usually twins, of flesh- 

 coloured or pink felspar, a dull white felspar, some greenish felspar, two 

 micas, quartz, and pyiite. Of this granite, four slices were prepared 

 by Mr. Jordan, of the Mining Record Office, two (B. 18 and 18*) from 

 a block of granite near Castletown, and two (B. 22 and 23) from the 

 rock at Crossfintan Point. 



B. 18. This slice is principally occupied by one of the large crystals 

 of flesh-coloured felspar. In the slice, the minerals observed with a 

 power of 17 are : an ii'regular skeleton of quartz, thi-ee varieties of 

 felspar, one crystal of amphibole, secretions of 

 mica, and a few opaque crystals (pyrite ?). 

 The large flesh-coloured crystal, which is pro- 

 bably orthoclase, has a lining structure that 

 runs obliquely from right to left (see woodcut), 

 but having darkly-shaded portions that extend 

 irregularly from left to right, while scattered 

 over it are numerous secretions of quartz. The 

 second felspar [dull white] is semiopaque, but 

 contains numerous iridescent spots ; this evi- 

 dently is a variety of orthoclase, and probably adularia. The thu-d 

 felspar only appears in small crystals, and portions of crystals ; it 

 shows minute parallel lines of prismatic colours, apparently being a 

 triclinic felspar. 



In this slice, most of the quartz occurs in the skeleton of the rock ; 

 however, a few particles were detected in irregular crystalline secre- 

 tions, somewhat like the quartz characteristic of elvanyte. The 

 micas, amphibole and pyiite (?) usually occur together, forming 

 irregular nests ; they are, however, sometimes in scattered small 

 patches, while a few individual flakes or crystals were detected. 



B. 18*. This slice seems to show the general character of the rock 

 in mass. With a power of 17 the three varieties of felspar appear to 

 be nearly equally developed. The quartz, besides forming the skele- 

 ton of the rock, appears in distinct crystals, while associated with the 



