﻿390 



MR. T. 0. BOSWORTH ON METAMORPHISM [Aug. 1910, 



occur in the knobs is cut across, a group of cross-sections of the 

 ^component slender prisms is seen. Each of these crystal-prisms 



Fig. 9. 



gives a ' diamond - shaped ' cross- 

 section ; and they are set close 

 together, with their corresponding 

 sides approximately parallel or even 

 in contact, so that the whole group 

 behaves almost as a single crystal. 

 The largest cross-section observed 

 measures about "01 inch across, and 

 the angles are about 70° and 110°. 

 No pinakoids are developed, but 

 traces of a strong pinakoidal clea- 

 vage cross the sections parallel to 

 the longer diagonal. This cleavage- 

 plane is also the plane of the optic 

 axes, for the cross - sections give 

 good interference-figures, disposed as 

 shown in the accompanying fig. 9. 

 Hence this is the plane usually 

 •taken as (010) in sillimanite ; and, 

 accepting the figures a:b = -970 : 1, it follows that the longer 

 ■diagonal in these crystals is along the a axis, and the prism here 

 •developed is of the form (2, 3, 0). 

 The interference-figure gives 



c — y ; a = a ; b=j3; and 2E = 45° approximately (measured 

 with an eye-piece micrometer). 



The crystals are thus positive, and have o and (3 very nearly 

 equal. The interference-colour seen in the cross-sections is an 

 extremely low grey. But y is considerably greater, for* in long 

 sections the interference-colours are often of the second order. The 

 refractive index y is slightly greater than that of a-monobromo- 

 naphthalene (1*658). The crystals appear very clear, but with a 

 quarter-inch objective they are in many cases seen to enclose 

 grains of green spinel, sometimes in great numbers, together with 

 some small particles of iron oxide. 



The andalusite is associated with the sillimanite in smaller 

 amount in the lenticles occurring in the rock, but it was not found 

 on crushing up the weathered-out knobs, having possibly been 

 weathered away. The crystals are often of large size [13971], and 

 enclose some patches of quartz and numerous brown biotite-crystals ; 

 but they are not in the least clouded with small inclusions, being 

 quite clear and of a strong pink colour in parts. 



The cordierite, which is plentiful in most of the rock-sections, 

 occurs in clear medium- sized grains, similar to the grains of quartz 

 and felspar. It is generally in the mosaic with these minerals, 

 and so closely resembles the quartz that it is with difficulty distin- 

 guished therefrom. The characteristic greenish pleochroic halos are 



