250 



B. KOTO. 



a qiiartzite, being- essentially made up of quartz grain?* together with 

 thin layers of a brown biotite and silky mica. Under the microscope 

 the schist is seen to be don il t np of quartz, biotite, feldspars in small 

 quantity, and lastly, long, thin, lamellar, colourless crystals resolving 

 at both ends into stiif fibres. The last-mentioned mineral is travers- 

 ed by fissures at various points, arid is not beut and lamellated like 

 ordinary micas, showing thus its brittle nature. In the rock-mass 

 this com])onent is approximately arranged in a parallel direction ; 

 between crossed niçois it extinguishes light parallel to the bmger 

 side; its polarization-colour is vivid, coming near to that of musc(nite, 

 but it may be easily discriminated by the want of nacreous lustre, and 

 by a peculiar wavy structure when vie^ved between crossed niçois. I 

 conjecture this mineral to be margarite. Siauroscopic examinations 

 show on a cleaved face a wide optic angle, so much so that the cen- 

 tres of concentric rings scarcely lie within the microscopic field. The 

 dispersion of the optic axis for the red is smaller than that for the 

 violet ; i. e., /"C^, wiiile that of muscovite would be just the reverse. 

 The biotite is oftheu.sual brown colour, and is interlaced with the 

 lamellse of margarite ; it is found in far less quantities than the other 

 mica. The quartz fills up the interstices of the other components, 

 and is consequently wedge-shaped ; it shows intense polarization- 

 colours. Occasionally grains of feldspars may be noticed. 



A c) (Tarnet-Biotiie-Sdnst. 



There is no rock whicli varies so greatly in its (jutward appear- 

 ance as the garnet- biotite-schist. This is in the main due to the 

 varvinii" quantity of biotite. The one extreme is a highly quartzose 

 schist of a greyish colour, with its characteristic vitreous lustre, and its 

 weathered portion usually coloured brown. Micaceous zones are 

 indicated by dark stripes on the transversely fractured surface of the 



