﻿122 



MR. G. W. TYERELL OjS t THE 



[vol. lxxii, 



(5) Asymmetry of the Sill. 



When the arrangement and petrographic nature of the various 

 layers composing the sill is examined in detail, a decided asymmetry- 

 becomes apparent. The bands, while generally strictly parallel to 

 the contacts, are not repeated in the same order at the top and 

 at the bottom of the sill. The upper layer of teschenite, becoming 

 richer in analcite downwards, comes to an abrupt end at a sharp 

 junction with fine-grained theralite. The lower hiyer of teschenite 

 likewise passes very rapidly into the base of the peridotite stratum, 

 although the actual junction is everywhere obscured. The thera- 

 lite band consequently does not appear in the lower half of the sill 

 (fig. 4, p. 96). 



The asymmetry is much accentuated if the densities and sizes of • 

 the various bands are taken into consideration. The heaviest and 

 largest layer, the peridotite, is not arranged centrally, but is 

 so situated that its centre-line is well below the geometrical centre- 

 line of the sill. On its upper margin it is flanked by a big mass 

 of the somewhat less heavy picrite and theralite, and on its lower 

 margin by a considerably lighter and smaller band of teschenite. 

 Thus the centre of mass of the whole sill must be considerably 

 below its geometrical centre-line. This, of itself, suggests that 

 gravity must have been the controlling factor in the arrangement, 

 at least in the central ultrabasic stratum. The densities of the 

 layers, bearing out the above facts, are shown in the vertical 

 section (fig. 4, p. 96). When plotted, they form a curve which 

 bulges in an asymmetric manner below the centre-plane of the 

 sill. 



(6) Density-Stratification in the Sill. 



Apart from the outer sheath of teschenite and its contact-facies, 

 the remainder of the sill, constituting the central ultrabasic 

 stratum, has itself suffered a subsidiary gravity-stratification. 

 This is indicated by the distribution of olivine, which has collected 

 in the lower layers of the mass. The upper portion of the stratum 

 consists of theralite with 14 per cent, of olivine. This passes 

 gradually downwards into picrite with 30 per cent., and finally into 

 peridotite with 65 per cent, of olivine. The inference is that 

 olivine, the earliest constituent to crystallize, has sunk under the 

 influence of gravity to the lower levels of the stratum. This is in 

 accordance with the experiments of Dr. N. L. Bowen, 1 who found 

 that olivine-crystals, forming in an artificial melt approximating 

 in chemical composition to a basic igneous rock, segregated in a 

 dense layer at the base of the crucible in which the melt was 

 contained. 



1 ' Crystallization-Differentiation in Silicate Liquids ' Amer. J ourn. Sci. 

 ser. 4, vol. ssxix (1915) pp. 175-91. 



