﻿130 MK. E. H. RASTALL ON THE [May IO/IO, 



schistosity-planes of the rock, but at right angles to them. The 

 pleochroisra is strong (reddish brown to colourless) and the 

 birefringence rather low (y-a=*02 approximately). 



Kg. 2. — Diagrammatic sketch of the right lank of the upper- part 

 of the Glenderamacltin valley. 



-Bed of stream x = Chiastolite. c = Cordierite spots. 



Certain clear areas, circular or oval in form, and free from 

 crystals of mica, are obviously incipient crystals of cordierite, and 

 in many cases faint indications of the usual complex twinning can 

 be seen in them. The optical characters of the cordierite will be 

 dealt with later, in connexion with specimens in which the crystals 

 show a more complete development. 



The rare crystals of chiastolite are easily discriminated from the 

 cordierite by their more definite outline, greater clearness, and 

 freedom from inclusions. They now consist of a micaceous or 

 •chloritic substance of moderate birefringence, occurring as 

 aggregates of minute flakes, and showing a general structure which, 

 when examined under a high power, strongly recalls serpentine. 



Associated with this type are certain bands of a more gritty 

 character, consisting for the most part of minute grains of quartz. 

 In these the same brown biotite is shown, but cordierite appears to 

 be absent : this is what would naturally be expected. 



The most interesting feature of the specimens from this locality 

 is the clear evidence that they afford of incipient crystallization : 

 the cordierite is obviously developing at the expense of the white 

 mica of the original slate, but it is not clear what mineral or com- 

 bination of minerals gave rise to the brown mica. In this low 

 grade of alteration new white mica is not developed to any extent ; 

 but, under a high power, occasional flakes can be seen enclosed in 

 the brown mica, and often in parallel intergrowth with it. 



The rocks of this type never come into contact with either of the 

 larger visible masses of granite ; therefore, as an example of the 



