428 A POPULAR EXPOSITION OF THE 
cones, and of a grey, black, green, brown, or varied colour, and break- 
ing into sharp-edged fragments, they constitute the variety properly 
called Obsidian. Pearlstone is a closely-related variety, containing 
small spherical concretions of a more or less pearly aspect. Another 
form, of a black, dull-red, green, or dark colour, and of a somewhat 
pitch-like aspect, is called Pitchstone or Retinite. This latter is 
stated by Sir. W. E. Logan to occur on the north shores of Lakes 
Huron and Superior. It should be observed, however, that the term 
Retinite is applied by some authors to a bituminous substance of a 
very different character. 
3. Trachytes.—These rocks have normally a harsh, rough tex- 
ture (whence their name from tpaxvs), and a white or light colour ; 
and they are either entirely or essentially feldspathic in composition. 
They offer three principal varieties, exclusive of Pumice, which may 
.be placed either here or amongst the lavas. These varieties merge, 
however, into one another. They comprise: common Trachyte and 
compact Trachyte, composed normally of Orthoclase or potash feldspar, 
and Granitic Trachyte, a rock of a granitic structure, made up of 
orthoclase feldspar, with small crystals or grains of hornblende, augite, 
ormica. Common Trachyte occurs chiefly in active or extinct volcanic 
districts. It often contains crystals of glassy feldspar, and sometimes 
scales and crystals of mica, &c. Occasionally, also, free silica or 
quartz is found in it, although accidentally, as it were, and only in 
small quantity. Compact Trachyte, or feldspar trap, as this variety 
has been termed, is found in broad straight veins or “dykes,” of a 
white colour, traversing the Montreal mountain, and occurring also 
(of a pale-reddish colour) at Chambly, &c. Granitic Trachyte (in 
some instances closely resembling granite, but differing from that rock 
by the absence of quartz) forms the eruptive mountains of Brome, 
Shefford, Yamaska, Rougemont, Belceil, Mount Johnson,’ &c., of 
the Eastern Townships. These granitic trachytes (or granitic dio- 
rites, as they might be termed with equal justice, see below) differ 
a good deal in colour and appearance, according to the amount of 
hornblende, mica, &c., which they contain. Like the compact 
trachyte of Lachine and Chambly, they are sometimes “porphyritic”’ 
—containing more or less distinct and large crystals of feldspar. (See 
Mr. Hunt’s Report for 1856, and that for 1858; also this Journal, 
Vol. V. page 426.) Many of the trachytes of these localities are in 
a partially altered state, effervescing in acids from the presence of 
