﻿part 2] 



PICBITE-TESCHENITE SILL OF LUGAB. 



121 



theralite, but was richer in magnesia, and poorer in lime, soda, and 

 alumina, indicating a greater richness in olivine-molecules and 

 poverty in those of felspars, as compared with theralite ; in short, 

 the magma must have been transitional towards the picrite. Con- 

 sequently, we arrive at the important conclusion that the original 

 magma had a composition different from that of the present 

 contact-rock, and hence, that the sill was heterogeneous when 

 emplaced, and has been shifted since differentiation. 



(4) Composition, Identity, and Banding of the 

 Contact-Rocks. 



Petrographical examination shows that the upper and lower 

 contacts of the Lugar sill are identical, even down to the smaller 

 microscopical features. Both consist of black, basalt-like rocks, 

 curiously streaked and drawn out in slightly-varying layers, and 

 injected by veins of coarse pink teschenite. They both pass 

 gradually into coarser and more normal teschenites towards the 

 interior of the sill. The upper teschenite becomes more analcitic 

 as it is traced downwards ; the lower teschenite remains compara- 

 tively uniform as it is followed upwards. These facts show that 

 the teschenite must have been injected into cold rocks, which 

 exercised the chilling effect proper to such contact. 



The banding is evidence of considerable movement of the 

 viscous magma during or after emplacement. That the movement 

 and banding took place prior to crystallization is shown by the 

 fact that the textures of the various types are perfectly granular. 

 There is no sign whatever of the parallel arrangement of columnar 

 or tabular minerals such as felspar or augite. 



As has been shown in the previous section, the chemical compo- 

 sition of the contact-rock is dissimilar to the average rock of the 

 sill, and consequently the facies cannot be considered as due to 

 the differentiation of an original homogeneous teschenite magma. 

 The fact has no significance if the hypothesis of successive intru- 

 sion be accepted. If, however, the sill be considered as due to a 

 single act of intrusion, its heterogeneity must have arisen prior to 

 emplacement. It would have originally had the composition of an 

 ultrabasic theralite or picrite, and contact-rocks of like composition 

 would have been formed. To account for the present disposition 

 of the facies it would be necessary to assume that the intrusion has 

 been moved on from its first position, where it w T as differentiated, 

 into the position that it now occupies, leaving its picritic contact- 

 rocks behind adhering to the old contacts. This view has some 

 support in the marked flow-banding of the contact-rocks ; but it is 

 difficult to understand why this movement did not more seriously 

 disturb the stratified arrangement of the various layers within the 

 sill. This assumption of movement of the sill as a whole, subse- 

 quent to differentiation, is not necessary if the alternative hypothesis 

 of successive intrusion be accepted. 



