426 A POPULAR EXPOSITION OF THE 
granitic aspect, geologically known as Gneiss. Some of the “ Thou- 
sand Islands’? consist of a very ancient sandstone. At Montreal, 
again, together with limestone, &c., we find in the picturesque Moun- 
tain, a dark and massive (or unstratified) rock, termed Trap, and 
more or less closely allied to the lavas of volcanic districts. These 
examples, without proceeding further, are sufficient to shew the diver- 
sity which prevails with regard to the rock-matters of comparatively 
neighbouring localities. But if we look, not to the mineral characters 
of these and other rocks, but to their respective origins or modes of 
formation—as evidenced by what is now going on in Nature in dif- 
ferent parts of the world—it will be fom that they fall naturally 
into. three groups, as follows : 
Eruptive Rocks. 
Meramorruic Rocks. 
SEDIMENTARY Rocks. 
In each of the above groups, the included rocks are of various 
periods of formation, as explained in the Chronological Classification 
at the close of the present Part of our Essay. Before proceeding, 
however, to a discussion of this question, and in order more especially 
to prepare the general reader for a proper understanding of Parr V, 
in which the geology of Canada first comes properly under review, it 
is necessary to consider these groups separately, and to enter into a 
few of their more practical details. 
ERUPTIVE ROCKS. 
The rocks of this division are of Igneous or of Aqueo-I[gneous 
origin. That is to say, their present form is due to solidification 
from a fluid or plastic condition brought about by the agency of heat, 
assisted, in most (if not in all) cases, by that of steam or heated 
water. They have been formed beneath the earth’s surface (whence 
the term “ Endogenous” applied to them by Humboldt), and they 
have been driven up or erupted, at various geological epochs, through 
cracks and fissures in the overlying rocks. They are distinguished 
by never occurring in true strata, but always in the form of irregular, 
protruded masses, which sometimes present a columnar structure, 
or in that of broad overlying or intercalated sheets, or in straight 
veins called ‘‘dykes”’ (see further on), or in ordinary tortuous veins. 
Secondly, by never exhibiting in their structure the marks of a sedi- 
