MICROPEGMATITE IN PURCELL SILLS. 
15 
origin and relations of these rock types will be given below under 
“Rock Genesis.” 
The exomorphic contact effect of the sills is very small, 
especially where the contact rock is a fine-grained argillaceous 
quartzite. In one case the quartzite for an inch from the upper 
contact is thoroughly vitrified and charged with small needles 
of hornblende. Outside this narrow contact zone, the sediments 
are normal in character. A study of the contact under the 
microscope revealed three distinct but gradational bands. The 
normal quartzite consists of interlocking grains of quartz, 0-5 
mm. in diameter. This zone passes gradually into the vitreous 
variety which shows no great signs of metamorphism, except a 
thorough cementing of the quartz grains, whose contact with 
each other is very indistinct and whose individuality was only 
detected by their optical orientation . Hornblende with green- 
ish-blue pleochroism parallel to c and identical with the horn- 
blende in the gabbro, is present in this vitrified band. This 
zone with a gradual increase in orthoclase and plagioclase passes 
into micropegmatite in which the quartz holds the feldspar, 
the latter being filled with dust-like inclusions of sericite. Horn- 
blende, similar to that described above, and biotite are sparingly 
present in this micropegmatite zone. As the hornblende and 
plagioclase become greater in amount, with a concomitant de- 
crease in the quartz and orthoclase, the micropegmatite variety 
passes into the hornblende-gabbro proper. All three zones were 
seen in one slide taken from a specimen at the contact. 
The exomorphic contact action, exhibited at the lower contact, 
was studied in two localities. In the Pyramid basin, a sill 150 
feet thick is intruded into grey weathering quartzites and 
argillaceous quartzites. The latter, at the lower contact of the 
sill, is impregnated at a distance of one foot with hornblende, 
similar to that found in the sill itself. This hornblende is 
shredded in appearance and sometimes occurs in radiating 
groups embedded in a fine ground-mass of quartz grains. Musco- 
vite in small, needle-shaped crystals is present in moderate 
amounts. A fine-grained variety of the quartzite occurred at a 
distance of 6 inches from the lower contact, and its metamor- 
phism consisted only in the development of a few crystals of 
