MINERALS AND GEOLOGY OF CANADA. 427 
mentary origin, such as rolled stones, grains of sand, &c. And, 
thirdly, by never containing organic remains. 
Where eruptive rocks traverse or lie in contact with other forma- 
tions, these latter are usually found to be more or less altered, as 
though by the agency of heat, near the points of contact. Coal-beds 
are thus for some distance often burnt into .cinder or converted 
into coke; soft limestones changed into crystalline marbles; sand- 
stones altered in colour, hardened, and changed into quartz-rock, and 
so forth. 
These rocks are arranged by Sir Charles Lyell in two broad divi- 
sions: Volcanic and Plutonic rocks; but it is impossible to draw a 
distinct line of demarcation between the two. Granite and syenite, for 
example, belong to the Plutonic series ; but certain granitic trachytes 
connect the granites with the volcanic rocks; and in like manner, 
certain greenstones merge on the one hand into syenite, and on the 
other (the distinction between augite and hornblende being now es- 
sentially broken down) into augitic lavas. This equally affects the 
sub-division into Volcanic, Trappean, and Granitic rocks, adopted by 
other observers. I would therefore propose, as an arrangement of 
convenience, the distribution of the Eruptive rocks into the six fol- 
lowing groups :—1. Lavas; 2. Obsidians; 3. Trachytes; 4. Traps 
and Greenstones; 5. Serpentines; 6. Granites. On each of these it 
is necessary to make a few observations. 
1. Lavas.—These comprise the actual rock-matters which issue in 
a molten condition from volcanic vents. During the solidification of 
lava currents, dense volumes of steam are emitted from the cooling 
mass. Lavas are of two general kinds: feldspathic, and feldspatho- 
augitic. The first, and by far the most common of the two, are 
composed essentially of feldspar, and are mostly of a dark or light 
grey colour. They pass into trachytes. The second are composed 
essentially of feldspar and augite,* are dark green or almost black in 
colour, and undistinguishable, except by their actual conditions of 
occurrence, from many traps or basalts. As examples of these rocks 
are not found in Canada, we need not describe their varieties, or enter 
into further particulars respecting their conditions of occurrence. 
2. Obsidians.—The rocks grouped under this division, are lavas, or 
other igneous products, in a vitreous or glassy state. They are en- 
tirely feldspathic in composition. When in connexion with voleanic 
* See descriptions of these minerals in Part II. of this Essay, 
MOL. VL. 2G 
