17

their outcrops and boundaries are not very clearly delineated. The alter-
ation and oxidation of the sills have in many cases so obscured their petro-
graphic character that it does not afford a clue to their intrusive relations.
Several sills of aplite or felsite are exposed at the head of the hydraulic pit
of the Waverly mine on Grouse creek. An accurate detail sketch of this
area was made in the field (Figure 1), which shows quite clearly the cross-
cutting relations of some of these minor intrusives. Contact metamorphism
of the adjacent slates and phyllites has produced along the borders of the
sills an assemblage of knots of cordierite which have been considerably
weathered to kaolin and limonite.

A noteworthy characteristic of almost all of these intrusives is their
irregular spotty replacement by siderite; and it is largely to the oxidation
of this mineral that they owe their typical brownish colour. In many
cases, as at the Waverly pit just mentioned, the felsite is so completely
replaced by siderite that specimens of it closely resemble ferriferous crystal-
line limestone. Small remnants only of orthoclase, acid plagioclase, and
muscovite remain as an indication of its original igneous nature. In other
places, the replacement by siderite is not so complete, and the true nature
of the rock can easily be read from a thin section. In these intermediate
or transition facies, the siderite appears in the thin section as if the rock
had been daubed with pale, buff-coloured paint, the daubs spreading
across several of the mineral grains irrespective of their character or shape.
In other localities dykes and sills of felsite and quartz porphyry occur in
which the replacement has been slight, and from these the petrography of
the intrusives is largely derived. Two of these occurrences are on Shepherd
creek and on the Kitchener claim on Proserpine mountain.

The primary minerals of the sills are quartz, orthoclase, and acidic
plagioclase (probably albite-oligoclase), occurring as phenocrysts and as
constituents of the groundmass, and muscovite in shred-like grains. One
slide contains a large phenocryst of hornblende and another shows micro-
pegmatite occupying the polyhedral spaces between the phenocrysts.
The groundmass is usually fine-grained and consists of a mixture of the
above-mentioned minerals. In places it is glassy with the development of
spherulites.

Many of the sills are seamed with a network of quartz veins, some of
which carry iron and lead sulphides with gold values. An excellent example
of this may be seen on the Dooley-Home Rule ledge on the southeast slope
of Barkerville mountain, where it is intersected by the old Goldfields.
hydraulic ditch.

Age and Correlation. It was found impossible to fix the date of intru-
sion of these sills and dykes. Neither they nor the quartz veins associated
with them were found cutting the Slide Mountain series, whereas pebbles:
of similar dykes and sill rocks were found in the basal conglomerate of that
series. All the available evidence, therefore, indicates that their intrusion
was pre-Mississippian.