PLATE XXXIII. 



FIG. 1. 

 AUGITE -GEANOPHYKE. 



BASE OF CRAIG CRAGGEN, MULL. 



Magnified 50 diameters. Ordinary light. 



The minerals represented are felspar (10), quartz (24), augite (7) and 

 magnetite (5). 



The felspar of this rock is mostly unstriated and often occurs as Carlsbad 

 twins. It is present as idiomorphic crystals and also as a constituent of micro- 

 pegmatite (10 and 24). The felspar of the micro-pegmatite surrounding a felspar- 

 crystal extinguishes simultaneously with the crystal, and when the crystal is a 

 Carlsbad twin the trace of the twin plane may be followed into the micro- 

 pegmatite. The quartz occurs as more or less independent grains and also as a 

 constituent of micro-pegmatite. The augite is green in colour and allotrio- 

 morphic. The grain in the centre is cut so that the principal cleavages intersect 

 approximately at right angles. It gives an optic axis in convergent light. 



Fid. 2. 

 CHIASTOLITE-SLATE. 



SKIDDAW, LAKE DISTRICT. 



Magnified 50 diameters. Drawn with polarizer only ; short axis right and left. 



The minerals represented are chiastolite (25), dark brown mica, white mica, 

 quartz, iron-ores. 



The figure shows one transverse and two longitudinal sections of chiastolite 

 prisms. The transverse section is not strictly at right angles to the vertical axis. 

 The characteristic inclusions of black pigment are seen in the central portions of 

 the section. 



The two micas occur both in large and small flakes. Quartz is present in 

 the form of irregular grains the boundaries of which can only be recognized 

 under crossed nicols. Iron-ores and possibly carbonaceous matter are represented 

 by opaque granules. 



The rock is a fossiliferous (Areiiigj slate altered by contact with granite. 



