INTRUSIVE ROCKS OF THE DISTRICT OF MONTREAL. 427 
like manner certain varieties gave to muriatic acid only traces of 
alumina from the decomposable silicate, which in other specimens 
equalled five or six per cent. and in one case from 36:0 to 46°0 per 
cent. and had the composition of natrolite, gelatinizing with acids ; 
the insoluble portion in this as in the other cases consisted of a feld- 
spar resembling orthoclase. This rock which contamed besides, about 
seven per cent. of carbonates, I described under the name of phono- 
lite. (Report for 1856, p. 490.) 
The feldspathic residue from these white traps contains from 60:0 
to 66°0 per cent. of silica, and only traces of lime, with from 10-0 to 
13°0 per cent. of alkalies, in which potash sometimes predominates, 
while more often soda makes up the larger portion, a fact observed in 
many orthoclase feldspars, especially those from trachyte: for to this 
class of rocks the white traps are for the most part to be referred, as 
already indicated by Sir W. E. Logan when describing as a trachytic 
porphyry, the feldspathic trap from Chambly, whose analysis is given 
at page 486 of the Report just cited. (See also Sir William Logan’s 
Report for 1847, p. 17.) ; | 
Under the title of trachytes, lithologists have included a large class 
of igneous rocks, generally more or less rough to the touch (as the 
name indicates,) white or of pale colors, and composed essentially of 
orthoclase or a closely related feldspar, with small portions of mica, 
hornblende and more rarely pyroxene. Some varieties contain dis- 
seminated grains of quartz. The typical trachytes have an uncrys- 
talline base, which is sometimes porous and at others compact, 
generally dull and earthy in aspect; the base is sometimes vitreous 
and passes into obsidian and pumice, while in others it is finely crys- 
tallme. These varieties often become porphyritic from the dissemi- 
nation of crystals of glassy feldspar and other minerals, passing 
into the so-called argillophyre or clay porphyry. The base is some- 
times highly silicious and becomes a sort of petrosilex, which is pro- 
bably nothing more than an intimate mixture of quartz and feldspar ; 
through such trachytes, and those which contain disseminated quartz, 
we have a passage to true granites, which consist of orthoclase feld- 
spar mingled with quartz and mica. There are not wanting trachytes 
whose whole mass is coarsely crystalline, constituting granitoid and 
even gneissoid trachytes. Such are some of the rocks about to be 
described, which are only distinguished from true granites and syenites 
by the absence of quartz. The analyses of other trachytic rocks 
