86 ME. C. E. TILLEY OJ^ THE GEAXITE-G]S'EISSES [vol. Ixxvii, 



The hornblende is associated with biotite. It is subidiomorphic, 

 optically negative, Z/\c = 16°, and shows well-developed amphibolic 

 cleavages. The cross-sections indicate that the forms (110) and 

 (010) characterize the prism zone. The pleochroic scheme is 



X = pale greenisli- yellow ; Y = brownish-green ; Z = deep green. 



These Lincoln gneisses (as already noticed) are by no means 

 imiform in chemical or structural composition. While potassic 

 microcline is usually the dominant felspar, various gradations are 

 met with, and the plagioclase member may exceed the felspar in 

 amount. 



Under the microscope the gneissic structure is indicated by the 

 parallelism of the elongate minerals biotite and hornblende, and 

 this structure may be developed even when the rock-mass in the 

 field shows but little evidence of fiow-structure. On the other 

 hand, types are met with which show no microscopical flow- 

 arrangement of the minerals, the structure being typically granitic. 

 Of the accessory minerals other than those already enumerated, 

 P3'rites and magnetite are the commonest, but occasionally an 

 almost isotropic allanite is met with. 



A microcline twinning is not a constant feature of the potash- 

 felspars, and in these cases an orthoclase is defined. A perthitic 

 character is common, and this (associated with the quadrille 

 structure) yields the typical microcline-microperthite. This micro- 

 perthitic structure is developed in two ways: (1) on a minute 

 scale, in which very fine lamella? of albite participate, and (2) in 

 the presence of spindle-shaped grains of greater dimensions. The 

 finer lamellse are arranged with respect to a cleavage-surface 

 (001) at an angle of 72°, as seen in sections parallel to (010). and, 

 as W, C. Brogger has shov/n, are parallel to the steep orthodome 

 (801). The spindle-shaped grains also are arranged parallel to this 

 surface ; but this development is less constant, and in some cases 

 their longer axes subtend the (001) cleavage, an angle more acute 

 of 63° or 64° corresponding to a linear arrangement parallel to the 

 vertical axis. Two such arrangements of perthitic inclusions have 

 been noted b}" Hennig, Siiess,^ and others. 



The plagioclase of the Lincoln gneisses shows a composition 

 varying from oligoclase-andesine to andesine, compositions more 

 acid than this being rarelv met with. 



Hordes of fine acicular to platy inclusions are often a cha- 

 racteristic of the plagioclase. The edges of these needles are 

 arranged along definite crystallographic directions. The edge of 

 one set is developed parallel to the 'a' axis of the plagioclase, with 

 two other sets running oblicjuely thereto. 



As to the nature of these fine inclusions, it does not a^^pear 

 possible always to differentiate rutile from ilmenite-rods^ for the 

 latter mineral becomes translucent in very thin films. There is 

 no development of the knee-shaped tmns so characteristic of the 



1 F. E. Suess, Jahrb. K. K. Geol. Eeiehsanst. vol. liv (1904) p. 417. 



