So-Callcd Basal Granite. — AlcCoiincll. 57 
Mineralogy. 
The principal constituents of the gneisses are quartz, ortho- 
clase, oligoclase and albite in varying proportions, and biotite. 
Hornblende is often present as well as biotite and in places the 
rock passes into a dioryte schist. The common secondary min- 
erals are epidote which is almost universally present, garnet 
abundant only in some localities and chlorite, muscovite and 
sericite. Bands of green, finely foliated chlorite schist, green 
and dark hornblende schist, and light grey silvery sericite and 
muscovite schists, occur both alternating with or fringing the 
gneisses, and in separate areas. These rocks in some places 
are so intimately connected with the gneisses, both structurally 
and mineralogicallv. that the inference is unavoidable that 
they ' simply represent different phases of a common magma, 
and this may be true in regard to them generally. The prin- 
cipal rock in the vicinity of Dawson is a light grey sericite schist. 
This rock as a rule is well foliated, and is often entirely re- 
crystallized, but in places enough of the original structure re- 
mains to show that it is derived in part at least, from a granite 
or quartz porphyry.* Similar rocks occur interbanded with 
the gneisses on Henderson creek, and at other places, and 
bordering them in the Fortymile district. These rocks have 
many characters in common with the gneisses, and while they 
may not in all cases be exactly contemporaneous, they evident- 
ly belonged to the same period of igneous activity, and for this 
reason have been included with them pendinir further study. 
Structure. 
The schistosity of the 2"neisses is far from uniform, the 
same area often including fine grained well foliated mica- 
schists partially or wholly recrystallized, ordinary gneisses, 
and coarse gneissoid granites showing little deformation. In 
some of the larger areas the schistosity becomes more marked 
towards the boundaries than at the center, but this feature ap- 
pears to be exceptional. The dips of the foliation are low as a 
rule, seldom exceeding 45° and probably averaging less than 
30°. High dips and sharp flexures do occur but are usually 
easily accounted for bv local causes. On the Stewart river 
the gneisses have been thrown, at their eastern boundary, into 
*A microscopical exaitiiiiatinn of this rock was made bv Dr A. E. Barlow 
and his conclnsions in regard to its origin agree with the field evidence. 
