﻿part 2] 



PICRITE-TESCHEISTTE SILL OF Lr/GAlf. 



123 



(7) Variations in Texture. 



Rapid variation in texture is common in alkali-rich rocks, and is 

 highly characteristic of the Lugar sill. These variations are doubt- 

 less connected with local variations in the physical and chemical 

 characters of the magma, especially in regard to the gas-content. 

 A textural feature is the comparatively fine grain of the theralite 

 stratum, which is poor in analcite. At the other extreme are 

 coarse-grained teschenites, especially those rich in analcite. The 

 controlling factor is probably the comparative abundance of fluxes, 

 especially water ; but the fine grain of the theralite may also be 

 partly due to chilling against the teschenite, as well as to com- 

 parative poverty of aqueous residuum. The peridotite is medium - 

 to coarse-grained, but this is assignable to the slow rate of cooling 

 m the centre of the sill. The dense teschenitie contact-facies are 

 clearly due to rapid chilling consequent upon intrusion into cold 

 rocks. The general granularity and lack of parallel orientation 

 among the crystals has already been commented upon. 



(8) Segregation-Veins. 



Many veins of a coarse pink teschenite cut the dense contact- 

 rocks, and show that a richly-analcitic magmatic residuum remained 

 in a liquid condition long after the consolidation of the contact- 

 rocks. Thin veins consisting largely of analcite and felspar also 

 cut the peridotite. Both types of vein doubtless represent a 

 slight pegmatitic phase in the development of the intrusion. These 

 veins are coarse-grained, and show no signs of chilling at their 

 margins. They were doubtless injected while the rocks were 

 still hot. There are also a few veins of a black fine-grained rock 

 resembling the teschenite-basalts of the contacts. These are 

 probably to be interpreted as injections of teschenitie material at a 

 later stage, when the contact-rocks were colder and able to exercise 

 a chilling effect. 



(9) Mode of Intrusion and Differentiation. 



Whatever hypothesis of intrusion be adopted, the main differ- 

 entiation of the Lugar sill must be referred to the stage of its 

 history prior to its emplacement. It is not possible, therefore, to 

 discuss it as fully as might have been done if it had occurred in situ 

 at a post-intrusion stage, with all the details clearly displayed. 

 The discussion of the petrography makes it clear that all the 

 different facies are genetically related, and it is highly probable 

 that they have arisen by the differentiation of a single body of 

 magma. 



The differentiation appears first to have produced bodies of 

 teschenite and an ultrabasic rock, which may be designated as 

 pier it e. 1 The mineralogical and chemical relations of these rocks 



1 The term picrite was first employed by Tschermak in the sense of a 

 melanocratic derivative of teschenite. 



