194 P^'of. Hull — On the Structure of Irish Granites. 



examples of trichites slightly bent or curved. The general appear- 

 ance of these in one part of the field is represented in Fig. 4. Some- 

 times the silica contains cells only without trichites.^ What the 

 nature of these needle-like objects may be I have no means of 

 judging from this slice. 



Cavities are exceedingly numerous in the silica. Some of these 

 resemble the forms described and figured by Mr. Sorby in his 

 admirable and well-known paper " On the Microscopical Structure 

 of Crystals." ^ They are of various shapes, inclosing the little 

 globule of fluid which just comes into view with a magnifying power 

 of 350 diameters ; and must be less than --^^-^ of an inch in diameter. 

 Others, however, are much larger, and sometimes do not appear to 

 contain any bubble ; and, as Mr. Sorby suggests, the fluid may have 

 escaped. One of the fluid cavities is represented in Fig. 5, others 

 in Fig. 4. 



Stone cavities, or appearances which I take to be such as figured 

 and described by Mr. Sorby, are also numerous in the silica. Along 

 with the confused broken materials which they contain are also 

 minute black specks. The form of these stone cavities is often very 

 irregular and ill defined. 



Occasionally perfect spheres occur, which may be assumed to be 

 gas cavities. They are, however, rare ; but one I have attempted to 

 represent (Fig. 6) is associated with several others, and is of the 

 apparent size represented, when magnified about 1000 diams. Figures 

 7 and 8 are similar spheres in the silica of a reddish porphyry from 

 the Old Eed Sandstone of Lesmahago, which are inserted for the 

 sake of comparison. They are exceedingly beautiful objects under 

 the microscope, the opaque walls shading off into the translucent 

 centres through a series of delicately blended purple rings. 



Felspar (Triclinic). — The dull waxy-looking felspar of the base is 

 triclinic, and in all probability oligoclase. With polarized light the 

 parallel-banded structure characteristic of triclinic felspars is ap- 

 parent in {he case of several small crystalline grains (see Fig. 9), 

 but the bands of colour are less vivid than in the case of some 

 albite felspars. I hope on a future occasion to compare the relative 

 refractive powers of albite and oligoclase in granite. 



Ortlioclase. — This felspar predominates, not only in the larger 

 individual flesh-coloured crystals, porphyritically developed, but in 

 the base itself. In the case of the larger crystals, however, it pre- 

 sents a very remarkable appearance, especially when seen by 

 polarized light, showing a croas-handed structure of a kind very dis- 

 similar from the banded structure of the triclinic felspars (Fig. 1.) 

 This structure I have observed in thin sections of granite from 

 other districts, such as Dublin and Wicklow ; but not in the felspar 

 of any other rock but granite ; and my present impression is that 



1 I am uncertain whether to use the term "belonites," "trichites," or "nadel- 

 c]ien" — terms used by Ziikel and Allport for somewhat similar ol)jects in the base of 

 some basalts. Apparently similar hair-like bodies from the "sanidinite" of Mont 

 Dore are considered by Dr. A. v. Lasaulx to he hornblende. Poggen. Annalen B. 

 144, p. 156. 



2 Quart. Journ. Geol. Soc., Lond., vol. xiv. p. 453. 



