522 MK. A. MARKER ON ROCKS 



submetallic lustre, and the groundmass, also stained with brown, 

 has a dull ap})eurance. Visible porj)hyritic l'el«pars are found, but 

 only very sparingly. The boundaries are often rounded, as if by 

 magmatic corrosion. Occasionally a crystal of orthoclase shows u 

 quite glassy lustre, but the felspars are usually dull and semi- 

 opaque. Certain rocks, such as the large dyke in Dry Sike and 

 the boss-like mass in Swindale, enclose grains of quartz of consider- 

 able size, the boundary rounded, or rarely with indications of the 

 dihexahedral form. The grains are commonly surrounded by a 

 thin coating of a dark green substance. 



Included fragments of partially vitrified grit, &c., are found in 

 the large Swindale boss. Vesicles, usually filled by calcite, also 

 occur. 



The mica of these rocks is a brown biotite in tabular crystals 

 more or less regularly bounded in the usual pseudo-hexagonal 

 fashion. The plane of the optic axes is perpendicular to the basal 

 cleavage and parallel to the clinopinacoid. The bisectrix makes a 

 very sensible angle (3" or 4^) with the normal to the basal cleavage, 

 so that in cross-sections of the fiakes the frequent twin-lamellation 

 parallel to the base is easily detected. Indeed, owing to the pleo- 

 chroism, this can be seen with a single nicol. Juxtaposition-twins 

 parallel to the prism-plane * are also common. In almost every 

 case the deep brown colour characteristic of biotite in most rocks is 

 here confined to the border of each fiake, the interior being much 

 paler, or, indeed, for vibrations parallel to the a-uxis, sensibly 

 colourless. Whether this is an original zonary structure or a result 

 of internal bleaching is not quite clear. Kosenbusch t apparently 

 takes the former view^ for the mica of lamprophyres generally. It 

 is noticeable that, although in cross-sections of flakes the border is 

 usually very sharply defined, basal sections show a more gradual 

 passage from dark to pale. Streaks of darker colour, following the 

 basal cleavage-direction, are occasionally seen passing through the 

 pale interior, and this seems rather to favour the idea of secondary 

 bleaching. Much rarer than the dark border is a dark nucleus, 

 always sharply defined by crystallographic planes, with a paler 

 margin [915]. The dark and pale mica-substances in these rocks 

 possess very different powers of birefringence. Rough measurements 

 gave the figures 0-06 and 0-04, which, according to Levy and 

 Lacroix, correspond to brown biotite and colourless meroxene 

 respectively. 



Resorption-phenomena, with a separation of iron ore, are some- 

 times seen on the edge of a flake [914]. Again, owing to mechani- 

 cal forces, the flakes have in some cases yielded along " gliding- 

 planes," as in the artificial twin-lamella? produced in calcite, cV:c. 

 The gliding-planes do not coincide with the basal cleavage [914]. 

 The mica occasionally encloses grains of magnetite, or is penetrated 

 by slender hexagonal prisms of apatite. Again, the edge of a fl;.ke 



* See Levy &,Lacroix, ' Miueruux des Eoclies ' (1888), p. 239, fig. 137- 

 t ' Mikr. Physiogr. d. mass. Gest.' 2nd ed. (1887) p. 310. 



