Professor Sull — On Irish Granites. 3 



to this observer, a spiecific grarntj of 2-6S6, and affords the following 

 chemical analysis.^ 



Silica 70-82 



Alumina 14-08 



Peroxide of iron 3'47 



Lime 2-65 



Magnesia 0-31 



Potash 4-64 



Soda ... _._. 2-31 



Loss by ignition 1"39 



99-67 



The admirable investigations of Dr. Haughton leave little to be 

 added in the way of analysis. Nevertheless, the microscope reveals 

 the presence of a second variety of felspar besides the orthoclase, 

 which he and Professor Galbraith were able alone to detect from 

 chemical examination. This second mineral, indeed, might probably 

 have remained unobserved, except for the introduction of this new 

 process of investigation.^ 



The rock from which the thin slices are taken is finely crystalline 

 granular, and consists of a siliceous paste, inclosing white felspar, 

 silvery-grey and black mica. "With polarized light the felspar is 

 seen to consist of orthoclase, and a triclinic felspar, less abundant 

 but well defined ; and the proportions in which the minerals occur 

 may be thus arranged : — 1. Silica; 2. Orthoclase; 3. Triclinic felspar; 

 4. Grey mica; 5. Black mica — the last occurring of a rich bronze 

 colour. 



Silica. — The silica occurs in an amorphous state, inclosing the 

 other minerals, as is usual in true granites ; but is itself composed 

 of numerous individual patches, each refracting the light differently, 

 so as to represent, on rotating the analyzer, a great variety of rich 

 colours. The individual patches are generally separated by a fringe 

 of prismatic hues, probably due to varying thicknesses of the section 

 at the edges. 



With the :^-inch object-glass numerous cells of varying shapes 

 and sizes come into view, together with some wonderfully long 

 "trichites" (" trichiten," Zirkel) (Fig. 13), some straight, others 

 bent or curved, and stretching through the mass in various directions. 

 They are quite distinct from the tubes, and I have no good idea 

 regarding their nature. 



The cells are often disposed in lines, or along planes, perhaps 

 concealed cleavage-planes. The cells for the most part contain fluid 

 bubbles (Fig. 7), but they are only visible with a very high power; 

 some, however, are " stone-cavities." 



Along with the cells and stone-cavities are also to be observed, 

 with a high power, very remarkable straight tubes (Figs. 8, 9, 10), 

 terminated by rounded ends, and evidently hollow, or filled with 

 gas. One of these (Fig. 10), remarkable for its length, seems to 

 contain a very minute bubble near the centre, and another at the 

 end of the tube itself. When first observed, I was at a loss to 



^ Haughton, "Granites of Ireland," Quart. Journ. Geol. Soc. vol. xii. p. 177. 

 . 2 Dr. Westropp, however, has detected a crystal of albite in the granite belonging 

 to the same mass at Dalkey. Journ. Geol. Soc. Dub. vol. ii. new ser. p. 213. 



