THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE II. VOL. I. 



No. I.— JANUARY, 1874. 



I. — Microscopic Structure of Irish Granites. 



No. 2. Granite of Aillemore, Co. Mayo. 



By Prof. Edward Hull, M.A., F.R.S., President R.G.S.I. 



(PLATE I.) 



[Read before the Eoyal Geological Society of Ireland, 11th November, 1873.] 



THIS granite forms an isolated mass, rising into two eminences a 

 few miles south of Lonisburg, called Corvock Brack (1287 feet) 

 and Knockaskeheen (1288 feet). It is a greyish granite — generally 

 fine-grained — consisting of quartz, two felspars, — one orthoclase, the 

 other triclinic, probably oligoclase — and dark green mica. In some 

 places there are patches in which the felspar assumes the appearance 

 of "graphic granite." Numerous boulders of this granite are strewn 

 over the district to the north-west, and on the south side of Knock- 

 askeheen ; the rock is traversed by regular joints ranging N. 10 W., 

 along which it splits off into nearly vertical walls. The position of 

 the granite is shown on Griffith's Geological Map of Ireland, and it 

 is surrounded by schistose beds, generally metamorphosed, and pro- 

 bably of Lower Silurian age. The granite itself is of older date than 

 the tipper Llandovery beds, which lie to the southward. 



Microscopic Slice. — The thin slice prepared by Mr. Jordan, of the 

 Mining Kecord Ofiice in London, shows the general structure of 

 the granite extremely well. With the 2-inch object-glass, and under 

 polarized light, its constituents are brought out in their relative 

 proportions ; the most abundant mineral being orthoclase, next silica, 

 then the triclinic felspar, then mica, and lastly magnetite. I shall 

 now describe these minerals in the order above stated. 



Orthoclase. — In the section this felspar is often clouded and 

 structureless, but is occasionally crystallized out into nearly perfect 

 crystals, in which the angle of 90° between the sides may be 

 frequently observed. But besides these there are several examples 

 of the "cross-banded" felspar, similar to that described and figured 

 in my paper on the granite of Firbogh. These come out under 

 polarized light, the bands crossing each other at angles approaching 

 90°. In one or two instances the banding takes a wavy form, and 

 is only in one direction (Fig. 4). These various forms of this 

 peculiar structure I regard as being characteristic of orthoclase, and 

 I showed in my former paper on the Firbogh granite that they 

 were due to a cellular structure along planes crossing each other 



DECADE II. — VOL. I. — NO. I. 1 



