GRANITE OP MrXLAGHDERG, CO. DONEGAL. 549 



considerably curtail its application as a decorative building-stone. 

 Its essential components are quartz, potash-felspar, soda-lirae-fclspar, 

 hornblende, and black mica, with sphene as an accessory constituent. 

 Since there is no white mica the rock must be classed with the 

 granitites. It is in fact a sphene-bearing hornblende-granitite. The 

 dominant felspar occurs in irregularly contoured masses, which are 

 sometimes of such considerable dimensions as almost to give the rock 

 a granito-porphyritic structure. The colour of this mineral, which 

 depends on the amount of weathering, determines the prevalent tint 

 of the rock. Reflections from cleaved surfaces show that almost 

 every crystal is made up of the two hemitropic components of a 

 Carlsbad twin. Its specific gravity, determined by means of 

 Sonstadt's solution and the Westphal's balance, is 2*57. These are 

 the figures assigned by Tschermak to a felspar of the composition 

 Org Abj*. 



Examined under the microscope, a considerable proportion of this 

 felspar is found to be microcline, presenting, between crossed nicols, 

 the rectangular intersection of spindle-shaped striae characteristic of 

 this mineral. In addition to this structure, suitably directed 

 sections (viz. those in the zone P : M.) show a system of small 

 irregular lamellae of triclinic felspar (albite or oligoclase), inter- 

 calated along the prismatic or macropinacoidal planes. To this 

 intergrowth of orthoclase or microcline with a felspar of the albite- 

 oligoclase series, F. Becke t has given the name of microperthite. 

 Irregular grains of striated felspar (oligoclase) also occur as 

 inclusions in the microcline ; less frequent are lamellae and grains of 

 quartz. This is probably the " quartz d' infiltration " of French 

 geologists. 



Felspar presenting no striated structure is also abundant. Such 

 felspar, according to the usual practice, is to be referred to ortho- 

 clase. A. Michel-Levy ij: has indeed made it appear likely that 

 orthoclase and microcline are identical, by showing that the optical 

 properties of orthoclase are such as would be expected to result 

 from an intimate intergrowth of microcline-lamellae on the albite- 

 and pericline-types. But the identity has not yet been proved. 



A curious phenomenon, which has been described and figured by 

 Becke §, is also well shown by an isolated section of this felspar. 

 This section is nearly parallel to the orthopinacoid, since it presents 

 the emergence of an axis of elasticity (a), and contains two sets of 

 fine, sharply marked cleavage-lines, crossing one another at an angle 

 of 106°. These are the cleavages of P and M, the divergence from 

 rectangularity being caused by the position of the section. An 



* Tschermak, " Die Feldspath-gruppe," Sitzungsber. der k. Akad. der Wissens. 

 Wien, Bd. 1. Abth. 1 (1864), p. 579. 



"Or" stands for one molecule of orthoclase (K2O, Al^Og (8103)6), and "Ab" 

 for one molecule of albite (Na^O, AI2O3 (Si02)6). 



t " Die Gneissformation des niederosterreichischen Waldviertels," Tscher- 

 mak's Min. und Petr. Mitth., Bd. iv. 1882. 



\ "Identite probable du microcline et de I'orthose," Bull, de li Soc. 

 Mineralog. de Fr. (1879), tome ii. p. 135. 



§ hoc. cit, p. 199. 



