GRANITE OF MULLAGHDEEG, CO. DONEGAL. 553 



cases prefers the t winning-planes. A few grains of unstriatcd felspar 

 are possibly orthoclase. Wedged in between the felspar-masses, 

 and evidently of later consolidation, is allotriomorphic quartz with 

 liquid-inclusions. Accessory grains of sphene are also peculiar to 

 this part of the spheroid. 



As the outer zone is approached, granules of the black opaque 

 mineral and plates of brown mica become more and more frequent ; 

 at the same time the quartz gradually ceases to occur. The bulk of 

 this portion of the spheroid consists also of plagioclase felspar, 

 diflFering from that of the nucleus only in the mode of arrangement. 

 For the most part, it appears, under the microscope, in wedge-shaped 

 sections, the longer sides of which converge towards the centre. Of 

 the remaining two sides, the broader base forms part of the external 

 periphery, while the other end terminates irregularly or unites 

 with the felspar of the nucleus. This radial orientation of the 

 felspar is distinctly indicated in certain parts of the section by 

 an approximate coincidence of the twin-striae (when viewed between 

 crossed nicols, or when marked by an alternation of turbid and clear 

 lamellae) with the direction of the radii of the spheroid (see fig. 4, 

 PI. XIV.). Such cuneate sections must represent rudely conical or 

 pyramidal masses of felspar which radiate from the nucleus, their 

 broad bases forming part of the external surface of the spheroid. 

 In the polished specimen the sections of the felspar-cones are easily 

 recognizable by the unaided eye. They are distinguished from the 

 intervening material by a faint chatoyant lustre, apparently caused 

 by reflection from internal surfaces. When the conical masses are 

 contiguous, the line of junction appears, in thin section, as a series 

 of steps, which are well brought out by the contrast of the different 

 polarization-tints. Cross-twinning, on the albite- and pericline-types, 

 is not unfrequent, the more persistent striation being, almost with- 

 out exception, in the radial direction. Many of the wedge-shaped 

 masses are, however, entirel}^ free from polysynthetic twinning. 



Scattered plentifully through the sections are grains of the opaque 

 mineral. These are of elongated form, being often 10 or 12 times 

 as long as broad, and show, here and there, octahedral terminations. 

 Disposed invariably with their long axes in the radial direction, 

 they appear, at first sight, distributed irregularly over the section. 

 A nearer examination, however, discloses the fact that, in more 

 than one instance, they are ranged, at equal distances from the 

 centre, side by side so as to produce the concentric marking referred 

 to above (see figs. 1, 3, PI. XIV.). This phenomenon clearly indicates 

 simultaneous separation of the opaque granules in concentric zones of 

 growth, the latter evidently proceeding from the central portion 

 outwards. 



More sparingly present are small plates of the brown mica. 

 This mineral shows strong absorption, and, since it possesses an 

 extremely small optic angle, may be classed with the biotites. Still 

 less frequent are scales of a green chlorite-like substance, resulting, 

 most likely, from the alteration of the mica. An isolated flake of 

 colourless muscovite was also observed. 



