THE SHA.P GKANITE AND ASSOCIATED KOCKS. 277 



Looking afc Lane's * two types of rock-forming sphene, it appears 

 that this corresponds to the type which he associates with rocks 

 poor in alkalies and rich in magnesia and iron-oxides. The sphene 

 in our rock is free from inclusions, excepting occasional crystals of 

 zircon and magnetite. The mineral sometimes occurs in granular 

 patches, but there is no reason to doubt that these also are of 

 original formation. 



Dark mica is the only ferro-magnesian silicate proper to the 

 rock. It forms moderately small flakes, which almost always, 

 when well bounded, show the pseudo-hexagonal appearance with 

 large basal plane. In connexion with certain marginal modifica- 

 tions of the rock, however, there occur larger plates of mica with 

 a different habit. These have the shape of long narrow blades, 

 bounded apparently by the forms c (001) and h (010), with irregular 

 terminations. They are often as much as an inch long, with a 

 breadth of less than j\j inch. When fresh, the mica is of a deep 

 brown colour with intense pleochroism, vibrations perpendicular 

 to the n-axis {i. e. nearly parallel to the cleavage-traces in a 

 section) being absorbed almost to opaqueness. The bisectrix is 

 not quite perpendicular to the basal plane, as may be verified 

 by a slightly oblique extinction in sections. This also enables 

 us to detect in some crystals a lamellar twinning parallel to the 

 base. 



The mica encloses occasionally any of the previously-named con- 

 stituents, besides its own secondary products. Its most usual mode 

 of alteration, exhibited in almost all the slides, results in a partial 

 decoloration, or more frequently a green colour in place of the brown, 

 and a considerable diminution in the absorption and pleochroism. 

 The process is efi^ected along the cleavage-planes of the mica, and 

 often gives rise to irregular lamellse of green colour alternating with 

 the brown t. A separation of granular magnetite invariably 

 accompanies this mode of decomposition. Side by side with flakes 

 so affected there are often others converted in their interior into a 

 reddish-brown substance free from magnetite. This shows less 

 intense pleochroism than the fresh mica, the absorption being 

 rather less parallel to the /3 and y axes, and greater parallel to a. 

 The optical properties are retained so far as to show the lamellar 

 twinning between crossed nicols, but the cleavage is obliterated. 

 Probably this represents a further stage of change than the green 

 mineral, the secondary magnetite having been reabsorbed in the 

 form of ferric oxide. The marginal parts of the flakes so afl'ected 

 are usually green, and still show cleavage-traces in the sections. 

 An examination of some basal sections of mica in the slides, or, 

 better, of thin films carefuUj^ flaked off from the mineral, frequently 

 reveals numerous minute needles of rutile disposed in three directions 

 parallel to the boundaries of the hexagon. These we have met with 

 only in the decomposing mica, and they may possibly be secondary 



* Tscherm. Min. u. Petr. Mittb. (N.S.) vol. ix. (1888) p. 207. 

 I- See Teall's 'British Petrography' (1888), pi. xxxv. fig. 1. 



