16 
MUSEUM BULLETIN NO. 2. 
biotite. Under the microscope the grains of quartz, whose 
average size is 0- 08 mm., shows a pitted structure as if acted upon 
by a solvent. Biotite is present in large amount. This differ- 
ence in degree and kind of metamorphism between these two 
laminae suggests that the texture as well as the composition had 
some influence on the contact metamorphism induced by the 
sill on the enclosing sediments. In the St. Mary sills, where 
a sill 140 feet in thickness was intruded into fine-grained argil- 
laceous quartzites, the contact effects are very slight and extend 
for a distance of 3 feet from the upper contact. Th^ only 
result, visible in the hand specimen, is a slight baking of the 
sediments. Under the microscope, this baking is seen to consist 
of a slight coalescence of the quartz grains whose average 
diameter is 0 • 09 mm. Muscovite, which, in general, is restricted 
to the contact metamorphosed sediments, is present in rod-like 
individuals. Biotite, common to all the sediments of the Aldridge 
formation, occurs in irregular masses 0-49 mm. in diameter. 
Summary of the Contact Metamorphism . — The sharp line of 
demarcation between the sill and the sediments is worthy of 
notice, for though, as the microscopical examination showed, 
there is a gradational change from sediments into granophyr, 
yet this change takes place in a zone with a width only a small 
fraction of an inch. The very small amount of contact meta- 
morphism induced by the intrusion is perhaps what might be 
expected, when the intruded rock is a fine-grained quartzite. 
It consists of a baking of the sediments for a maximum distance 
of 3 feet from the contact, with the formation of muscovite 
and the transference of femic constituents from the gabbro into 
the quartzites for the distance of 1 foot. 
THE STRUCTURE OF SILLS. 
The Moyie Sills . — One of the finest, as well as one of the most 
easily accessible examples of differentiated and undifferentiated 
sills, is exposed on the western slope of the mountain west of 
Kingsgate, B.C., on the International Boundary line. This 
section was described in some detail by Daly l . 
J R. A. Daly, Summary Report, Geol. Surv. Can., 1904, p. 98A. 
R. A. Daly, Amer. Jour. Sei., 4th Series, vol. 20, 1905, p. 185. 
R. A. Daly, Festschrift zurn Siebzigsten Geburtstage von Harry Rosenbusch, 
1906, p. 203. 
