MICROPEGMATITE IN PURCELL SILLS. 
21 
of the sill schlieren of acid material were observed. Sill “B” 
is separated from the underlying sill “C” by 200 feet of argil- 
laceous quartzite. Sill “C” is 123 feet thick and consists 
entirely of hornblende gabbro. The two remaining sills observed 
in this section were respectively 565 and 2,165 feet in thickness, 
but as they were not well exposed, several bands of sediment 
may be contained in each of the two sills. No granite (micro- 
pegmatite) was found in, these sills. 
ROCK GENESIS. 
The facts which any theory as to the origin of the granite 
(micropegmatite) in the Purcell Sills must account for, are as 
follows : — 
(a) The majority of the sills are entirely basic and non- 
stratiform. 
(5) Some of the sills are composite. 
In connexion with the composite sills, the following facts 
are noted: — 
(1) The presence or the amount of granite (micropegmatite) 
in these sills bears no relation to the thickness of the sills. 
(2) The composite sills are stratified according to density. 
The following theories might be advanced to explain the 
above facts. — 
(1) The simple sills may be magmatically distinct from the 
composite sills. 
(a) If magmatically distinct, no special explanation is 
necessary for the “simple” sills. In the case of the “composite” 
sills any of the alternatives given in “(2)” might hold. 
(2) The simple sills and the composite sills are magmatically 
related. 
(a) The composite sills were injected along with a 
simple magma which gave rise to the granite (micropegmatite) 
because of : — 
(1) fusion and recrystallization. 
(2) assimilation. 
(3) differentiation. 
