174 



MR J. HORNE AND MR J. J. H. TEALL ON BOROLANITE. 



Sphene is by no means uniformly distributed in the different varieties. In the 

 specimen from the north-west slopes of Cnoc-na-Sr5ine, to which reference has already 

 been made, it occurs in large ophitic plates which are allotriomorphic with respect to 

 felspar and pyroxene. In the melanite-bearing rocks sphene is frequently present in 

 the form of minute ('03 x '07 mm.) and often spindle-shaped granules. These granules 

 are found only in the garnet. They sometimes occur so abundantly as to leave scarcely 

 any of the isotropic garnet -substance between them in the thin sections. At other times 

 they are entirely absent. That they are sphene is proved by the fact that they possess 

 the refraction, double-refraction, colour, pleochroism and dispersion of this mineral. 



Apatite is present in the form of stout hexagonal prisms. It is always perfectly 

 fresh, and may occur as inclusions in any of the other constituents. In one exceptional 

 specimen from the burn at Ledmore it is present in great abundance. This specimen is 

 a black, coarsely crystalline rock composed of pyroxene, melanite and apatite, with a 

 little biotite and pyrite. 



Magnetite is sparingly present in many of the rocks. It occurs as grains which may 

 be readily extracted from the powder of the rock by means of a weak magnet. 



The felspar of these rocks is frequently associated with a turbid substance giving 

 indefinite optical characters. In one or two instances this substance shows hexagonal (see 

 fig. 5, PI. XXXVII.) and rectangular sections. As a rule, it either forms mi cro-pegmatitic 

 aggregates with felspar, or occurs in patches with no suggestion of crystalline form. On 

 treating a slide or a cut surface of the rock with hydrochloric acid, little protuberances of 

 gelatinous silica mark the distribution of this substance. The acid solution contains 

 soda in abundance.* 



It seems impossible, therefore, to avoid the conclusion that nepheline occurred as an 

 original constituent of these rocks. This conclusion is strengthened by the fact that 

 melanite and green pyroxenes are well-known associates of nepheline and leucite. 



A peculiar blue substance occurs wedged in between the large individuals of 

 orthoclase in certain veins, and is found also as a constituent of some of the white spots. 

 It shows aggregate polarisation, and is decomposed by hydrochloric and sulphuric acids, 

 with the separation of gelatinous silica and the evolution of bubbles. 



After adding water to the hydrochloric acid solution and evaporating slowly, salt and 

 gypsum crystals are developed — the former in great abundance. A partial analysis was 

 made on about half a gramme of this substance, with the following result : — 



Silica, 



Alumina, 



Lime, 



Potash, 



Soda, 



Sulphuric anhydride, 



Sp. Gr., 



2-41-2 



43. 



36-1 



28-4 



3-2 



1-8 



16-2 



5-9 



916 



* Proved by the uranium-acetate test. 



