MINERALS AND GEOLOGY OF CANADA. 115 
into three groups, viz :—(a) Gneissoid strata ; (6) Limestones, Quartz- 
ites, and Iron bands; and (c), Anorthosites or feldspar rocks. 
(a) Gneissoid Strata :—These make up the larger mass of our Lau- 
rentian rocks. Ordinary gneiss, as explained in Parr III., consists of 
‘quartz, potash-feldspar, and mica ; whilst in syenitic gneiss, the mica 
is replaced by hornblende. These varieties occur both alone and 
mixed with one another, throughout our Laurentian districts. The 
feldspar is generally red or white, the quartz colourless and vitreous, 
and the mica and hornblende of some dark tint—black, brown, or 
green. ‘The two latter minerals occasionally die out, when a binary 
mixture of quartz and feldspar results. In certain beds of coarse 
‘structure, the stratification lmes become obscure; but usually, and 
‘even in hand specimens, gneiss exhibits a striped or banded aspect, by 
which it is distinguished from ordimary granite. The potash-feldspar 
or orthoclase (see Part II.) is sometimes replaced or accompanied by 
soda-feldspar or albite, but the instances of this are not common. The 
predominating colour of these gneissoid strata, is reddish or dark grey, 
the latter resulting from stripes of dark mica combined with narrow 
‘zones of white quartz and white or pale red feldspar. When much 
hornblende is present, the rock may assume a black or dark greenish 
colour, or present a fiecked surface of red and black : exhibiting in the 
former case, a transition into hornblende-rock. A red gneiss with 
green layers of epidote, forming a stone well adapted for ornamental 
‘purposes, occurs at Carlton Place near Kingston, and at some other 
localities. The black or dark-green hornblende-rock associated with 
the gneissoid and limestone strata, frequently contains crystals of red 
garnet (Barrie township, &c.) ; and the latter mineral sometimes occurs 
in the gneiss or quartzites in considerable abundance (Grenville, River 
Rouge, &c.) It is usually found, however, in the vicinity of the lime- 
stone bands, occasionally forming true garnet-rock. 
(b) Limestones, Quartzites, and Tron Ores:—The limestone beds 
associated with the gneissoid and other Laurentian rocks are often of 
a fine granular or saccharoidal texture; at other times coarse granular, 
and occasionally almost compact. The colour is usually white or grey, 
but for short distances the rock is sometimes of a pale reddish, or 
greenish tint. It is frequently zoned with specks and scales of mica, 
serpentine, or graphite, and contains also various accidental minerals, 
of which the more abundant or interesting comprise: Iron pyrites ; 
apatite or phosphate of lime; sulphate of baryta; tremolite, diopside, 
