KiNAHAN — On Microscopical Structure of Hocks. 161 



XXI. — Report on the Miceoscopical Stetjcthre of Eocks. 'No. 2., 

 By G. H. KiNAHAN, M. R. I. A., &c. (With Plate 8.) 



[Read February 8, 1875.] 



On the Quartz contained in the Granites from Knockanavaddy (-5^), 

 BaUynahown (-S'), Furiogh {£^ and B*), and KirhuUen {B^), County 

 Galway. 



The quartz in these granites occurs principally as the skeleton of 

 the rocks ; but a small portion is found as inlying blebs and crystals, 

 principally in the orthoclase. The latter variety is very similar to the 

 quartz blebs so characteristic of the elvanytes (PL 8, fig. 19). 



The blebs of quartz in the elvanyte- as a rule are in crystalline 

 forms, or coated particles ; some, however, are in irregular pieces, 

 partaking more of the character of the skeleton quartz of typi- 

 cal granite. When typical they have sharp outlines, but in some 

 the margin seems to gi^aduate qtiickly into the surrounding minerals. 

 Many of them contain a "well-developed complete crystal of a 

 foreign mineral (PL 8, fig. 19). All are affected more or less 

 by gas specks or laheculce. These, however, as a rule, are not 

 numerous ; and in those blebs in which many are found, the quartz 

 occurs in a more or less irregular form. Short gas tubes, or tuhuli, 

 seem to be characteristic of the quartz of the elvanytes. In the right- 

 hand quartz crystal (PL 8, fig. 19),uncler apower of 169, very few /aSecw/cg 

 appear, nor are they much increased in number under a power of 386. 

 In the left-hand crystal, under a power of 169, there are also very few 

 labeculae to be seen ; but under the higher power (386), about double 

 the quantity appears, some of the latter being so small as to be scarcely 

 visible, while in two places they foKU short strings of minute beads. 

 In both crystals short thick tubuli occur. These, under a power of 196, 

 appear as short thick lines ; but with higher powers (fi-om 250 to 400) 

 they are seen to be miaute tubuli, probably fonned by gas. Pig. 23, 

 PL 8, represents a grotq) of them that occui's in the right-hand crystal. 

 The two shai'p cones are tubuli which lie oblique to the plane of the 

 section. Some of the labeculae, especially most of those that are com- 

 plete spheres, seem to be gas bubbles; but many of the others, 

 especially those that are oval and irregular in outline, would appear 

 to be cross sections of the tubuli ; their different shapes being due to 

 the angles at which the tubuli are cut, and the refiection of the light 

 against the walls of the tubuli. The crystals without shai'p margin 

 (left-hand fig. 19, PL 8) usually have more or less regular fringes of 

 prismatic colour's. 



The skeleton quartz in the granites B', B^, B^, and B* has no 

 crystalline forms, and gives a gorgeous change of colour when 

 the polarizer is turned, the different masses of colour generally 



