180 



Proceedings of the Eoyal Irish Academy. 



XXVI.— IXGENITE 



EocKs. Eeport iS'o. 4. By G. H. Krs'AHAir, 

 M.E.I.A., &c., (Src. 



[Read May 10, 1875.] 



Lo^'GSTOI^E, Co. Tipperary, Ordnance Sheet 58. — An intrusire mass, 

 coming up tlu'ongh. the lo-^er-becMed carboniferous limestone. This 

 granitic rock is a more or less granular elvanyte, which may be thus 

 described : — a slightly, granular pui'plish brown color, weathering to a 

 dii-ty reddish or orange. The base contains crystals of yellowish-greenish 

 felspar, some small blebs of quartz, nests of minute greenish flakes 

 (mica?), wliich congTegate in a matiix that weathers ferruginous. In 

 this protrusion there has not been a deep quarry opened, and, as the 

 rock is more or less weathered to a depth of over twenty feet, it is im- 

 possible to prociu'e a normal specimen of the rock, and, as may be 

 expected, the slices cut are unsatisfactoiy, as the minerals they con- 

 tain are more or less affected by rust stains. Under a power of 33 the 

 rock seems to have a brownish felspathic matrix, containing numerous 

 black, opaque, ill-defined crystals ; and the latter, under a high power 

 (327), are found to be ciystals of ppite changed on the edges into 

 rust. In the matrix also are found numerous specks and small se- 

 cretions of quartz ; the latter, under a power of 258, are found to be 

 aiTanged somewhat similar to what is shown in figs. 1 & 2, where the 

 unshaded portions are the quartz ; the obliquely-shaded portions, the 

 felspathic matrix ; the long crystals, appear to be amphibole, while 

 there are bunches, or individual crystals (some well-marked octohe- 

 drons), of pyrite, or perhaps of a fernferous garnet. 



Fior. 1. 



Fig. 



The mass of the quartz evidently has crystallized out after the 

 other minerals, as it is found filling up the vacancies. In it were re- 

 marked lines, groups and scattered minute air bubbles, and some tubuli; 

 in the sections examined they were not very numerous. In one piece 

 of quartz forming a triangle (fig. 2), the plane is traversed by nume- 

 rous irregular lines, giving the crystal a ruptured aspect (fig. 4) ; but 



