The Granite Industry of Southwestern New Brunswick: A Historical ...

Part One — The St. George District 5
with the ‘red granite,’ which has been quarried far more extensively.
The chief minerals are striated feldspar, pyroxene and hornblende,
with considerable biotite. Thin sections from a few localities show
that the feldspar is mainly labradorite, that most of the hornblende
is secondary after pyroxene (augite), and that in some localities
olivine is one of the constituent minerals.
Parks (1914, p. 146) classified the rock as ‘diabase’ because of
the ophitic texture, but the coarse texture and schiller structure of the
olivine, coupled with the fact that many typical gabbros show ophitic
texture, seem to justify the terms ‘gabbro’ and ‘olivine gabbro.’
The gabbroic rocks are believed to be older than the St.
George granite for the following reasons. First, the black granite
in the Frost Mountain Quarry (Map 2, 47) and Mears Quarries
(53) is cut by dykes of granitoid rock, which may be offshoots from
the St. George granite (Map 2). Second, the St. George granite
becomes finer grained near its contact with the gabbro. And third,
gabbroic dykes have not been found cutting the St. George granite.
The gabbro is also believed to be intrusive into sedimentary
rocks of Silurian age. If these conclusions are correct, the intrusion
probably took place during a period of deformation in the Late
Silurian or Early Devonian. [Ed. note: McLaughlin et al. (2003)
have age-dated the gabbro as 421 ± 4 Ma or Late Silurian.]
The Stone
The stone quarried in the St. George District falls into two
general classes: the ‘St. George Red Granite’ (Parks 1914, p. 110–
122) and ‘St. George Black Granite’ (Parks 1914, p. 145–151).
In the industry they are referred to under the trade names of Red
Granite and Black Granite or St. George Red and St. George Black.
They are referenced hereafter simply as ‘red granite’ or ‘black granite.’
Red Granite
The red granite is made up chiefly of potassium feldspars
(orthoclase and/or microcline), plagioclase and quartz with small
amounts of biotite. The hue is due to red colouring matter in the
feldspars. In some instances, all of the feldspars are red, giving a
deep red colour to the whole rock. In others, only some of the
feldspars are coloured, and there are all variations of combinations,
down to stone in which the feldspars are nearly all flesh-coloured or
white. On this account, the red granite varies in colour from place
to place, ranging from deep red through light red, salmon, pink,
flesh-coloured and ivory to almost pure white.
Certain colours may be obtained from the different quarries,
giving a wide range of selection. The deeper shades of red and pink
have been the most popular in the past, but there is a growing
demand for the lighter shades of pink and rose in the building
trade. It is interesting to note that the deepest reds have so far been
found along the southern edge of the granite.
Examination of four thin sections of light-coloured to deep
red granite shows that the rock is made up chiefly of orthoclase
and microcline, quartz, striated feldspar and varied amounts of
pleochroic brown biotite. The reddish colour is confined almost
entirely to the orthoclase and microcline, but the colouring matter
is too fine to be identified under the microscope. Orthoclase and
microcline vary in the amount of alteration from almost clear in
the tinted granite to almost opaque in the deep red varieties; the
colour of the stone appears to be determined entirely by the colour
of the feldspar.
A striated feldspar, probably albite, is a common constituent.
It occurs as individual crystals more or less altered to sericite,
sometimes with a clear border. It also occurs as distinct microscopic