﻿part 2] PICRITE-TESCHE1UTE SILL OP LUGAR. 95 



A movement of the magma has given rise to distinct schlieren, 

 distinguishable by slight differences of colour and texture, at both 

 contacts. The same explanation can hardly be applied, however, 

 to the remarkable differences obtaining in the interior of the sill. 

 Including the contact-basalts and the normal teschenites continuous 

 with them, the rock of the interior is divided into at least three 

 ■different bands by some process of differentiation or by successive 

 intrusion. First there is a band of ultrabasic rock— picrites and 

 peridotites of coarse texture and remarkable freshness — occupying 

 the major part of the interior, and indeed of the whole mass, and 

 resting on the lower teschenite. The picrite forms the upper part 

 of the ultrabasic stratum and the peridotite the lower. Above the 

 picrite comes a band about 10 to 15 feet thick, of a fine-grained, 

 basic, nepheline-rock belonging to the theralite family, which may 

 be considered as continuous with the picrite and passing into it 

 somewhat rapidly. Between the theralite and the normal teschenite 

 overlying it generally intervenes a thin and variable layer of highly 

 analcitic rock, patches, streaks, and veins of which, or a rock allied 

 thereto, permeate the theralite in its immediate vicinity. Fig. 4 

 (p. 96) embodies a vertical section of the sill showing the approxi- 

 mate thickness of the various facies, and the map (fig. 3, p. 94) 

 illustrates the surface-distribution of the facies in a limited area 

 at the confluence of the Bellow and Grlenmuir Waters. 



The differentiation will be dealt with in greater detail, after the 

 •discussion of the microscopical and chemical evidence. 



III. Petrography. 

 (1) The Contact-Rocks. (PI. X, fig. 1; PI. XI, fig. 4.) 



The rocks of the upper and lower contacts are identical in all 

 respects, and will therefore be described together in this section. 

 They consist, for the greater part, of a hard, dense, basaltic rock, 

 usually black or dark grey. In several specimens bands or schlieren 

 ■differing in many subtle gradations of colour and texture are 

 seen. Thin veins of coarse flesh-coloured teschenite, indifferently 

 traversing all the schlieren, are rather numerous. 



Microscopically the rock is holocrystalline, on the whole very 

 fine-grained, and shows numerous sharp variations in texture and 

 composition. The minerals observed are plagioclase, augite, biotite, 

 analcite, magnetite, and occasionally olivine. The chemical 

 analysis (p. 104) shows that a considerable amount of orthoclase 

 must be present. A thin section of a rock obtained at the lower 

 contact in Grlenmuir Water gives a good general idea of the 

 aspect of both contact-facies, and will accordingly be described 

 in detail. There are several distinct types of rock in the slide 

 (PI. XI, fig. 4). 



(a) Starting from one side of the section there is first a schlieren composed 

 of an even-grained aggregate of plagioclase-laths, subhedral prismoids of 

 jmrplish augite, numerous small ragged plates of reddish biotite enclosing all 



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