﻿part 2] PICRITE-TESCHENITE SILL OF LUGAE. HI 



rr T, he T^°f eC ! tneSS of the reference of the first two rocks in 

 lable IV to the theralites, and the general accuracy of the Rosiwal 

 analyses, is shown by the accordance of the calculated analyses 

 with the analysis of Rosenbusch's type theralite from Duppau 

 (Bohemia) The excess of magnesia and the slight deficiency 

 m lime of the Lugar theralites are due to their more highly mafic 

 character ; but the alkalies, alumina, and silica correspond remark- 

 ably well with those of the Duppau rock. 



Turning to the lugarite, the calculated Rosiwal analyses corre- 

 spond well with the chemical analyses, except in lime, potash, and 

 phosphorus pentoxide. The excess of lime and phosphorus pent- 

 oxide is partly due to the over-estimation of apatite, and partly to 

 the overlooking of orthoclase and its measurement as labradorite. 

 Ihe deficiency of about 1 per cent, in potash is, of course, due 

 directly to the latter cause. The lugarite analyses are characterized 

 by a very large content of alkalies along with a comparatively- 

 low silica percentage, and by their persodic character. They fall 

 into the hitherto unoccupied and unnamed persodic subranP- of 

 lujavrase (II.7 1.5) of the American Quantitative Classification.! 



Systematically, these rocks may be described as ijolites in which 

 the place of nephelme is partly taken by analcite, and in which 

 barkevikite occurs as well as, and sometimes to the exclusion of 

 augite. One of the analyses of ijolite from livaara (Finland)' 

 corresponds fairly well with that of lugarite (Table IY No VII) 

 It is, however, richer in lime and alkalies than the Lugar rock 

 and contains much less combined water, since nepheline and not 

 analcite is the principal felsic mineral. 



The only other rocks hitherto described with an analcite-content 

 of tne i same order as that of lugarite are the heronite * of Heron 

 ±Jay Lake Superior, with 47 per cent, of analcite ; and the anal- 

 cite-tmguaite of Pickard's Point, Essex County (Mass ) with 37"4 

 per cent of analcite. The chemical analyses of these rocks are 

 set out for comparison in Table IV. Heronite differs from lugarite 

 m containing a arge amount (28-2 per cent.) of orthoclase, which 

 is reflected m the large potash-content of the analysis, although 

 the soda-content compares well with that of lugarite The 

 analcite-tmguaite of Pickard's Point contains segirine and anortho- 

 clase phenocrysts m a ground-mass composed of nepheline and 

 analcite, is richer in alkalies than lugarite, and is devoid of lime- 

 soda felspar. 



(5) Picrite and Peridotite. (PI. XI, figs. 2 & 3.) 



The ultrabasic rock which makes up more than half of the Lugar 

 sill is characteristically felspar-free, and is a typical hornblende- 

 peridotite. Some varieties, however, occurring towards the top of 



I £• W - T 7 rr ell, Geol. Mag-, dec. 6, vol. ii (1915) p. 361. 

 ' NerlhpW* AllT f g ^ Gd a / a decom P° sed tinguaite ; see A. E. Barlow, 



it£:.L^tsir^ Port Coldwel1 C0nt - ] ' Guide - book m - 8 



K 2 



