28 
MUSEUM BULLETIN NO. 2. 
action of gravity. This latter explanation is more applicable to 
differentiation in sills on account of their lateral extent. In the 
case of the Shonkinsag laccolith, there is direct evidence that the 
syenitic facies is not due to assimilation in place of the sur- 
rounding Cretaceous sediments, for the composition of the 
laccolith is dominately alkaline, while that of the enclosing 
sediments is salic. Another example called the Lugar sill is 
described by Tyrrell. 1 This intrusive mass 140 feet thick, 
enclosed by sandstones and shales, differentiated according to 
density into the following zones : — 
Rock type. 
S.G. 
Upper part 35 feet thick . . 
Teschenite, coarse, plus analcite 
2-64 
“ normal 
2-70 
“ camptonitic, 
2-98 
Monchiquitic 
2-99 
Central part 51-5 feet 
thick 
Picrite. 
301 
Lower part 17 * 5 feet thick 
Teschenite, camptonitic 
2-81 
“ normal 
2-71 
Tyrrell excludes assimilation in the formation of the above 
magma. This seems perfectly logical from the low percentage 
of silica and the presence of analcite in the resulting rocks. 
In the Purcell sills, it is probable that assimilation of some of 
the enclosing quartzites took place, but assimilation of this kind 
is held to be a minor factor in the formation of the granite 
(micropegmatite) in the composite sills. Occasionally blocks 
of quartzites were found in the sills. When these xenoliths 
occur in the granite, they are charged with needles of horn- 
blende which greatly resemble the hornblende of the gabbro. 
No contact aureole of micropegmatite was found around these 
blocks. In the gabbro the xenoliths only suffered a baking, 
he contact between the quartzite and the gabbro being sharp 
^Tyrrell, G. W., Trans. Geol. Soc. of Glasgow, vol. 13, 1909, p. 298. 
