﻿part 2] 



PICRITE-TESCHENITE SILL OE LTJGAE. 



127 



occupied by olivine-crystals. Moreover, the iron-ores are sparsely 

 developed, and would be prevented from sinking to any great 

 extent because of the small size of the crystals. The augite, on 

 the other hand, forms large platy or columnar crystals, shapes, 

 however, which would not facilitate sinking as readily as that of 

 the more compact equidimensional olivine-crystals. Furthermore, 

 the augite tends to form subophitic aggregates with labradorite- 

 laths, a circumstance which would still further hinder sinking. In 

 view of these, considerations it is easy to see why olivine should 

 dominate the sinking swarm, and hence why the gravity-stratifica- 

 tion in the Lugar sill should be defined mainly by the relative 

 abundance of olivine-crystals. That augite has sunk to some extent 

 is shown by the existence of richly-pyroxenic layers in the picrite 

 part of the stratum, and by the general greater abundance of augite 

 in the picrite than in the overlying theralite (see Table VII, p. 117). 

 This is exactly where one would expect the augite to concentrate, 

 in view of the fact that it is the second heavy mineral to crystallize 

 in bulk. The lowermost kiyers, on the view adopted here, would 

 be dominated by olivine, a deduction matched by the presence of 

 peridotite in this position. 



The hypothesis of sinking of heavy crystals, which is be- 

 lieved to be well attested in the central stratum of the Lugar sill, 

 may be applied to the differentiation of the Lugar magma as a 

 whole. Teschenite and picrite may be regarded as the opposite 

 poles of a gravitative differentiation effected by the sinking of 

 heavy crystals in the magma-chamber whence the Lugar sill 

 proceeded. The teschenite, as the lighter differentiate, would 

 occupy the upper part of the reservoir, and thus, on the application 

 of stress, would probably be injected first, the picrite following as 

 the result of renewed stress. The sunken olivine-crystals might 

 be partly dissolved in depth, and the rounded condition of the 

 crystals in the Lugar peridotite may be cited in favour of this 

 conclusion. 



The slight concentration of analcite in the upper teschenite at 

 the junction with theralite (fig. 4, p. 96), may be accounted for 

 by a little settling of heavy crystals in the teschenite, prior to the 

 intrusion of the picrite, aided, perhaps, by a concomitant upward 

 movement of the light aqueo-alkaline material which solidified 

 as analcite. The later history of the sill is mainly that of con- 

 tinued crystallization. After the crystallization and sinking of 

 olivine-crystals had well progressed, the augite, hornblende, iron- 

 ores, and felspars crystallized almost simultaneously, leaving a hot, 

 chemically-active, aqueo-alkaline residuum which finally crystallized 

 as analcite and alkali-felspar. The formation of analcite in this 

 way opens up some interesting magmatic and mineralogical 

 problems, which I do not propose to discuss in this paper. 1 The 

 presence of this residuum is attested by the numerous veins of 



1 See A. Scott, ' Primary Analcite & Analcitization ' Trans. Geol. Soc. 

 Glasgow, vol. xvi (1916) pp. 32-43. 



l'2 



