66 
TUE GEOLOGIST. 
Referring to the agents wliicli, in the interior of the earth, may have 
aided in rendering rocks plastic, or generally tended to the develop- 
ment of minerals, he reduces them to four — heat, water, pressure, and 
molecular action, attributing to the fii'st only a comparatively limited 
role in the formation of eruptive rocks. 
After describing what he conceives to be the action of these 
various agents, he thereon founds a classification of all eruptive 
rocks, according to their mode of origin, into the following divisions : 
I. Igneous eruptive rocks. 
II. Pseudo-igneous eruptive rocks. 
III. Non-igneous eruptive rocks. 
I. Igneous are those which have been reduced to a state of fasion, 
or at least rendered plastic by heat. They are almost always 
completely anhydrous, with a cellular stinicture and a rough feel to 
the touch, their constituent minerals having a strongly-marked cha- 
racteristic vitreous aspect ; they constitute the rocks regarded as 
eminently volcanic. As extreme types of this class he especially 
refers to, trachyte and dolerite.* 
II. Pseudo-igneous rocks are those that may have been reduced to 
a state of plasticity partly by igneous and pax-tly by aqueous action. 
Water, heat, and perhaps pressure may have combined to contribute 
to this. Rocks of this class have sometimes a cellular, or even 
scoriaceous structure ; but their constituent minerals have only a 
slightly vitreous aspect. They are hydrated, often containing 
zeolites, and dividing into prisms or spheroids, and are generally 
associated with igneous rocks in volcanic regions. As types of this 
class he refers to retiuite (pitchstone or phonoUte), basalt, and trap.f 
These two classes represent those rocks usually called volcanic. 
* Trachyte is a rock containing orthose and anorthose felspar, ferro-magnesian 
mica, hornblende, and also quartz. By anorthose felspar ia designated, in a 
general way, all the felspar species belonging to the sixth crystalline system. 
Dolerite is an anorthose and anhydrous lava, composed of anorthose felspar and 
augite, with sometimes olivine, mica, and leucite. 
f Eetinite consists of vitreous orthose felspar, of ferro-magnesian mica, and 
also quartz rather rarely, and always a large proportion of water, which may rise 
from 10 to 100 per cent. Peohstein and phonohte are varieties of this rock. 
Basalt principally consists of anorthose felspar, augite, and olivine, with some- 
times protoxide of iron, carbonates, zeolites ; and accidentally nepheline, haiiyne, 
zircon, corundum, &c., forming a hydrated felspathic paste. Basalt has the 
