﻿part 2] PICRITE-TESCHENITE SILL OF LUGAK. 109 1 



Apatite in needles and small crystals is extremely abundant, and 

 occurs embedded in all constituents save ilmenite. The longer 

 needles and prisms show the usual cross-fracture, and are frequently 

 bifid at the terminations. 



It is difficult to study the ground-mass, because of its turbidity : 

 it is composed mostly of alteration-products, a fine brown dust, 

 brightly-polarizing zeolites, and 'ghosts' of analcitized felspars ; but 

 occasionally it clears to an area of recognizable anal cite. Under a 

 high-power objective the fine brown dust resolves itself into clear 

 highly-refracting granules, embedded in a colourless isotropic 

 substance which has a refractive index distinctly lower than that 

 of Canada balsam, and, on the thin edges of the section, a cubic 

 cleavage. It is fairly clear, therefore, that the bulk of the ground- 

 mass is a cloudy analcite. Faint, streaky, paler patches, with 

 an occasional approach to hexagonal or rectangular outlines, are 

 probably to be referred to nepheline. This mineral occurs much 

 more recognizably in a lugarite-like rock in association with the 

 bekinkinite of Barshaw, near Paisley. 1 



Two varieties of lugarite are distinguished. In one, titanaugite 

 and barkevikite are developed in about equal proportions (Table III, 

 col. iii). The distribution of these minerals is, however, very 

 patchy. Some slides contain titanaugite or barkevikite only ; 

 others contain both. The veins that penetrate the picrite have 

 barkevikite only (Table III, col. iv), the prisms of which are 

 frequently arranged in a rude stellar fashion. 



Quantitative Mineral Composition of Theralite 

 and Lugarite. 



These were estimated by the Rosiwal method, and gave fairly 

 concordant results, which are recorded in Table III, below. 



Table III. 





I. 



II. 



III. 



IV. 



Plagioclase 2 



23-3 



16-4 



10-5 



14-6 



Analcite 







| 42 5 



|49-0 





12-6 



16-6 



Titanaugite 



36-1 



35-9 



217 









12-2 



17-2 



' 29-5 





18-6 



87 









3-6 



67 









4-2 



2-5 



5-0 



2-7 





1-6 



1-0 



3-1 



4-2 



I. Theralite, Bellow'Water, Lugar. 

 II. Hornblende-theralite, Bellow Water, Lugar. 



III. Lugarite, main mass, Glenmuir Water, Lugar. 



IV. Lugarite, veins in picrite, Glenmuir Water, Lugar. 



1 G. W. Tyrrell, Geol. Mag. dec. 6, vol. ii (1915) p. 308. 



2 Ab x An x in theralites ; AbjAn 2 in lugarites. 



Q. J. Gr. S. No. 286. k 



