14 
MUSEUM BULLETIN NO. 2. 
Hence, in the quantitative classification, the rock is of the 
class persalane, order columbare, rang albachase, and subrang 
albachose. 
Internal Relations . — The folding and faulting, to which the 
sills have been subject, is evidenced by the attitude and the 
distribution of the sills now exposed in the Purcell range. As 
they were intruded when the strata were flat, they have suffered 
all the movements which have taken place in that range, so that 
now they form anticlines and synclines with all angles of dip 
from 0 to 90 degrees. The sills often end abruptly against 
strata which are older or younger than those holding the sills, 
and in some cases the vertical displacement may be several 
thousands of feet. Columnar jointing, perpendicular to the 
upper and lower contact, is especially prominent in the thick 
sills and is well shown in the escarpment to the north of St. 
Mary lake. The cross-section of the columns is an acute angled 
quadrilateral. 
The most striking phenomenon in the internal structure of the 
sills is a stratification of the material according to density. 
The stratification is of two kinds. In the example, studied with 
Daly on the International Boundary line, the distribution of 
material was: an upper gabbro zone 26 feet thick passing grad- 
ually downwards into a granitic phase 80 feet thick, which in 
turn gradually passed into a lower gabbro layer 30 feet thick. 
This type of differentiation is similar to that at Shonkinsag 
described by Pirsson. 1 
The other type was studied in the St. Mary sills and consists 
of an upper granitic layer 70 feet thick passing gradually down- 
wards into a gabbro zone also 70 feet thick. All gradations 
exist between the granite (micropegmatite) and the gabbro, 
and an arbitrarily chosen type, representing the intermediate 
rock between the two extremes, is called the quartz-diorite. 
The thick basic sills also show a rough stratification in the centre 
of their masses, where long schlieren of acid material are elon- 
gated parallel to the contacts of the sills. The gabbro at the 
contacts of these basic sills is usually fine-grained, while in the 
centre it is coarse-grained and pegmatitic. A discussion of the 
*L. V. Pireson, U. S. G. S. Bull. 237, p. 43. 
