﻿Vol. 66.'] METAMOEPHISM ABOUND THE BOSS OF MULL GRANITE. 389 



As I do not find any record of this latter from the rocks of the 

 Scottish Highlands, I venture to describe it further. 



On examination of thin sections and of fragments obtained by 

 crushing, it is seen that the stout prisms in the weathered-out 

 knobs are not single crystals, but each is built up of a number of 

 more slender crystalline prisms packed together with almost, hut. 

 not quite exactly, the same optical orientation. 



Fig. 8. — Section of a lenticle, magnified 23 diameters \_1J/.125\ X 

 showing mainly long sections of sillimanite. 



T. C. H. 1909. 



Crystal fragments giving perfectly uniform extinction were- 

 obtained, measuring about an eightieth of an inch across. They 

 are positive in character, and show well-marked cleavage-traces or 

 a fibrous structure in the prism-zone and some irregular cross- 

 fractures. In the rock-slices single crystals were observed measur- 

 ing up to a quarter of an inch in length. 



Parallel growths of andalusite with this mineral, as described by 

 Prof. A. Lacroix, 2 also occur, appearing in section as longitudinal 

 strips of andalusite between strips of sillimanite. 



Wherever one of the rough stout prisms of sillimanite which 



1 For these three niicrophotographs (figs. 6-8) I am indebted to Mr. T. 

 C. Hall, F.G.S., who took great trouble to rephotograph with the identical 

 magnifications the exact fields shown in inferior photographs of my own 

 perpetration. 



2 ' Mineralogie de la France & de ses Colonies ' vol. i (1893) p. 42 & fig. 21. 



