MICROPEGMATITE IN PURCELL SILLS. 
5 
origin is supported by the association of the rhyolites and basalts 
in the Purcell Lavas, the supposed extrusive phase of the Purcell 
Sills. 
PETROGRAPHY OF THE SILLS. 
As mentioned above, the sills fall into two groups — a simple 
type composed almost solely of gabbro, and a composite type 
consisting of granite (micropegmatite), quartz diorite, and 
gabbro, always arranged in a stratiform fashion. These com- 
posite sills are of two types; in some cases the granite (micro- 
pegmatite), occupies the upper portion of the sill, grading 
downwards into gabbro (Sudbury type); while in the other type 
the granite (micropegmatite) is near the upper contact and 
grades towards each contact into gabbro (Shonkinsag type). 
Thus, there are three main rock types in the composite sills, 
and one rock type in the simple sills. Of these, gabbro is com- 
mon to all types of sills. 
Lithological Characters . — The rocks which constitute the 
Purcell Sills vary in composition from a hypersthene gabbro 
to a very acid granite (micropegmatite), with members inter- 
mediate between these two extreme types. The texture of the 
sill rock varies from fine-grained to porphyritic. The granite 
(micropegmatite) is always associated with gabbro and occurs 
at or near the upper contact of the sills. The thickness of the 
granite (micropegmatite), which grades downwards into a 
hornblende gabbro, bears no relationship to the thickness of the 
sill. In some cases, where the granite (micropegmatite) is not 
at the immediate upper contact, it grades upwards into a 
quartz diorite and finally into a gabbro. 
Before proceeding to the general discussion as to the origin 
and distribution of the granite (micropegmatite) in the sills, 
a description of the three main rock types will be given. It 
must be remembered that all three types pass into each other 
by gradual transition. 
Gabbro . — The most basic type is hypersthene gabbro and 
occurs in one of the St. Mary sills. It is a dark grey, crystalline 
rock of granitic texture, in which can be detected plagioclase and 
augite. The rock is quite fresh, the feldspars being clear and 
