and Igneous Rocks of Ensay. 85 



parts. This sample shows clearly that the pinite of this 

 vein has, in fact, resulted from the alteration of cordierite. 



In order to gain some further insight into the composition 

 of this pinite, and into the mode of its occurrence in this 

 vein, I prepared a thin slice. Under the microscope this is 

 seen to be composed of pinite individuals, quartz, and some 

 alkali-mica. The pinite has, in some instances, when seen 

 by polarised light, a peculiar meshed appearance, resembling 

 that of serpentine. I have observed the same in samples 

 of pinite from Bodenmais, in Bavaria, and Schneeberg and 

 Aue, in Saxony. This kind of alteration proceeds evidently 

 from the cleavages and cracks inwards. With a high power, 

 all that I could make out, in addition to that which I have 

 already said, was that some of the pinite is made up of 

 minute colourless flakes, which are in places contorted and 

 twisted together. In other places very numerous, yellowish- 

 coloured, thornlike microliths occur in the basal sections, at 

 all horizontal angles to each other. 



The pinite individuals are evidently not all in the same 

 state of change. In some there are still remaining portions 

 of the unaltered cordierite, whilst others are so completely 

 altered as to have only aggregate polarisation. In some 

 there are included grains of quartz or plates of alkali-mica, 

 and the latter almost always borders the pinite, and extends 

 beyond into flaws which traverse the quartz. Magnesia- 

 mica in chlorite pseudomorphs is only present in very small 

 amount. 



The quartz is full of fluid cavities, some of which have 

 small bubbles. 



The following is a quantitative analysis of a sample 

 collected from this vein : — 



No. 5. — Pinite. 

 Si.0 o ... ... ... 46-16 



A1. 2 3 ... ... ... 32-23 



Fe.O ° ... ... ... 334 



Mn.O ... ... ... -01 



Ca.O ... ... ... -50 



Mg.O ... „. ... 2-66 



K o 875 



Na.,0 ... ... .., -56 - 



H 2 ... ... ... 5-08 



99-29 



