172 



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



due rather to the introduction of carbonates from the surrounding limestone than to their 

 development by the decomposition of the rock. 



Nepheline almost certainly occurred as an original constituent of some varieties, but 

 is now only represented by decomposition products. Wollastonite is present as the 

 principal constituent of certain inclusions occurring in a specimen from the south side 

 of Sgonnan M5r. 



Orthoclase enters largely into the composition of all the rocks, and is found also in 

 certain veins. Tested by Szabo's method, the felspar of the rock appears to be identical 

 with that of the veins. The flame-reaction is that of an orthoclase fairly rich in soda. 



Fig. 2.— About Half Natural Size. 



The specific gravity is about 2*52. The felspar of the veins is of a purplish-brown colour, 

 and the individuals often measure an inch across. The two cleavages are easily 

 recognisable, but the basal cleavage is much the more perfect of the two. The reflections 

 from the basal cleavages are bright, those from the clino-pinacoid dull. The individuals 

 are tabular, with conspicuous development of the clino-pinacoid. Flakes parallel to 

 M {010} give extinction angles of 6° or 7° referred to the trace of the P {001} cleavage. 

 Those parallel to P sometimes give straight extinction and sometimes an indefinite 

 extinction due to different portions extinguishing in slightly different positions. In the 

 M-flakes the emergence of a positive bisectrix is seen in convergent light, and the 

 position of the optic axial plane can be proved to be that of normal orthoclase. The 

 twinning, when it occurs, is on the Carlsbad plan. In the massive varieties the 

 orthoclase occurs, as a rule, in large, allotriomorphic grains, forming, as it were, the 

 groundmass of the rock, the other minerals being present in it as inclusions. In the 

 foliated varieties it forms granulitic aggregates. Orthoclase forms a large portion of the 

 white spots, where it is often micro-pegmatitically intergrown with a substance which is 

 probably an alteration product after nepheline. 



Striated or plagioclase felspar is comparatively scarce and is entirely absent from 

 many varieties. It occurs as small irregular grains between large individuals of 

 orthoclase, as grains in association with similar grains of orthoclase, and also as a 



