96 DE. J. W. EVANS ON THE EOOKS OF THE [Feb. I906, 



Under the microscope, the felspar is seen to consist partly of 

 microcline and orthoclase, both with microperthitic inclusions on a 

 minute scale, and partly of albite, which sometimes contains more or 

 less rectangular inclusions of microcline or orthoclase arranged along 

 definite lines and extinguishing together. Quartz occurs in small 

 oval or circular blebs, such as are characteristic of granulites, or in 

 rounded lozenge-shaped individuals which appear to be sections of 

 the double pyramid, for the extinctions are parallel to the diagonals. 

 Other crystals are more or less hexagonal, and occasionally these 

 are dark in all positions of the nicols. These grains or crystals 

 of quartz are usually found as inclusions in felspars, or penetrating 

 into their margins ; while, in some instances, the felspar appears to 

 fill the irregular intervals between crystals of quartz, or the eroded 

 cavities in that mineral. There can be no doubt that the crystal- 

 lization of the quartz must, in all these cases, have preceded that of 

 most of the felspar. Micrographic structure, however, occasionally 

 occurs where the two must have crystallized together ; and there 

 are also cases where the quartz fills the interstices between felspars, 

 and must have been the last mineral to separate out. 



There is a fair amount of biotite and brown hornblende, which 

 is sometimes intergrown with quartz in a kind of micrographic 

 structure. I also observed a large, rounded, greenish-yellow, highly- 

 refracting isotropic crystal, 1*25 mm. in diameter, surrounded by 

 hornblendes, which is altered externally to a yellow decomposition- 

 product with low birefringence. The mineral is traversed by 

 cracks filled partly with a mineral of high birefringence, about 

 double that of quartz, and partly with fluor. It may, perhaps, be a 

 silicate of the rare earths allied to pyrochlore. 



A few grains of apatite, zircon, and magnetite are present. The 

 rock is rather fine-grained, the largest crystals, felspar and quartz, 

 not exceeding 3 millimetres in diameter. 



The foliation was not apparent under the microscope. 



In some bands or veins (M 1 b) the constituents approach a centi- 

 metre in diameter, and the dark constituents almost completely fail; 

 so that the rock may be described as a rather coarse-grained 

 haplite, consisting of quartz, a little microcline, and albite. In the 

 last are numerous minute plates, usually appearing like needles in 

 thin section. These are probably hydrous soda-mica, the result of 

 incipient decomposition. 



Elsewhere (M 1 a) there are dark bands in the gneiss that reveal, 

 under the microscope, an entirely- different composition. The 

 felspar is plagioclase, showing albite and frequently also pericline- 

 twinning in alternately broad and narrow bands. The maximum 

 angle of extinction with the lamellse of the albite-twinning, in. 

 sections perpendicular to the brachypinakoid, is 20°, and the refrac- 

 tive index is well above that of the slow (extraordinary) vibrations 

 in quartz. The felspar is, therefore, apparently an acid labradorite. 

 It contains numerous minute inclusions of hornblende, quartz, and 

 -apatite, and occasionally lines of rectangular isotropic inclusions 



