﻿part 2] PICRITE-TESCHENITE SILL OE LUGAB. 103 



difficulty of identifying and measuring potash-felspar, when occur- 

 ring in small quantities in rocks of this kind. Table I illustrates. 

 the mineral composition of a number of the Lugar teschenites. 



Table I. 





I. 



II. 



III. 



IV. 



V. 



Plagioclase (AbiArii) 



232 



27-9 



28-9 



34-5 



6-6 



Orthoclase 



10-2 



5-6 





2-9 





Analcite 



16-1 



19-9 



13-8 



lo'o 



12-2 



Titanaugite 



28-1 



27-2 



39-9 



27-3 



24-0 



Hornblende 











18-1 



Olivine (serpentine) 



10-6 



7 V 5 



4-6 



12-9 



32-5 



Biotite 



3'4 



1-5 



2-9 



2-1 



37 



Titaniferous iron-ore 



7-1 



93 



9-9 



37 



1-9 



Apatite 



1-3 



1-1 





1-1 



1-0 



I. Fine-grained banded teschenite, near the upper contact, Bellow Water. 

 II. Teschenite, rich in analcite, upper band of teschenite, Bellow Water. 



III. Teschenite, lower band, between peridotite and No. IV, Glenmuir Water. 



IV. Teschenite, near the lower contact, Glenmuir Water. 



V. Melanocratic teschenite, in the upper band, Bellow Water. 



The first four columns show that the mineral composition of the 

 Lugar teschenite is fairly constant. Labradorite averages about 

 28 per cent., analcite 15 per cent., titanaugite 30 per cent., olivine 

 8 per cent., biotite 2 per cent., iron-ore 7 per cent., and apatite 

 1 per cent. Orthoclase is a variable constituent, reaching 10'2 

 per cent, in No. I, and declining to nothing in No. III. It is 

 probable that a little nepheline occurs in some of the types, but it 

 is hard to detect and harder still to measure. It is probably 

 included with the analcite and orthoclase in the analyses. The 

 fifth analysis is clearly that of a highly mafic variety, with a large 

 increase in the proportion of olivine, and the incoming of 18 per 

 cent", of " hornblende, causing a concomitant drop in the proportions 

 of plagioclase and analcite. In respect of the proportions of light 

 and dark constituents (felsic and mafic, or leucocratic and melano- 

 cratic), it will be seen that there is a substantial ecpiality in the 

 first four rocks, that is, they are mafelsic. The fifth rock is 

 domafic. 1 



The Chemical Composition of the Teschenites. 



Two full chemical analyses of the Lugar teschenites were made 

 for me by Dr. A. Scott ; one of the upper contact-basalt, the 

 other of a normal teschenite from the upper band. These are 

 supplemented by analyses calculated from the mineral composi- 

 tions given in Table I. The minerals of variable composition 

 here are titanaugite," hornblende, biotite, and olivine. It was, 



1 For an explanation of these terms, see G. W. Tyrrell, 1 The Bekinkinite 

 of Barshaw (Benfrewshire) & the Associated Bocks ' Geol. Mag. dec. 6, vol. ii 

 (1915) p. 366. 



