﻿80 



ME. H. G-. SMITH ON THE 



[vol. lxxii, 



One of the earlier thin sections was seen to include a small patch 

 made up of an opaque mineral associated with elongated crystals 

 having a blue colour and a high refractive index. As there was 

 good reason to suppose that the blue mineral was corundum, it 

 was decided to make an effort to obtain additional examples. Two 

 methods were adopted. 



A considerable quantity of tbe rock was crushed and washed by 

 panning. The residue was divided by means of heavy liquids, the 

 last to be used being methylene iodide Avith a specific gravity of 

 3'3. Magnetite was extracted from the heaviest residue by means 

 of a bar-magnet, and the remaining constituents were examined 

 under the microscope. Pyrite, commonly as well-formed cubes but 

 occasionally crystallized as a pyritohedron, was the most abundant 

 of these minerals. Far less common were perfect crystals of zircon 

 and fragments of corundum and staurolite. Further particulars 

 concerning these minerals are given later. 



The second method employed was tbe cutting of slices with the 

 wheel. More than 150 of these were cut and examined. Some 

 of these were seen to contain black elliptical patches about a tenth 

 of an inch in diameter, and these slices, together with others not so 

 promising, were sectioned. Fifty sections were made, and of these, 

 twelve contained the minerals foreign to the rock. 



Three of these inclusions are selected for description. 



Inclusion I. (PI. ]X, e fig. 1.) — This consists largely of grains 

 of magnetite, which, for the most part, are black and opaque ; but 

 those near the margins of the inclusion are altered to a yellowish 

 oxide. Associated with the magnetite are many elongated crystals 

 transmitting a blue colour, which is irregularly distributed. Some 

 of the crystals are two or three times as long as broad, and have 

 oblique terminations ; mairy of them are almost acicular. They 

 have a refractive index slightly higher than 1*74, show cleavage 

 parallel to the length, and give a pleochroism of blue to colourless 

 with the maximum absorption for longitudinal variations ; they 

 polarize in first-order colours, show straight extinction, and have a 

 positive sign of elongation. Crystals which have been isolated are 

 tabular, and viewed at right angles to the tabular faces are isotropic. 

 One of them is bounded by sloping faces which focus alternately on 

 the upper and the lower surfaces, and there is no doubt that they 

 are faces of a rhombohedron. These isolated crystals give a uniaxial 

 figure, and the double refraction is negative. The conclusion is 

 that the mineral is corundum. These thin hexagonal tables, 

 combinations of basal pinacoid and rhombohedron, present points 

 of similarity to the more perfect and larger specimens from 

 Yogo Gulch (Montana) described by Pirsson 1 and Pratt. 2 



This mineral in the section is not in contact with the magnetite ; 

 in every case it is siu'rounded by a zone of colourless mica. The 

 inclusion is not bordered by any definite zone of minerals, but it is 



1 Amer. Journ. Sci, ser. 4, vol. iv (1897) p. 421. 



2 Ibid. p. 424. 



