466 
THE GEOLOGIST. 
nternal heat of the globe at the present epoch of its physical history. Their philosophy 
constitutes one great division of the science of Igneous Gkology. Plutonic action, 
an effect of the same cause, but deeply and entirely subterranean, the subject of another 
division of that science. These lectures appropriated chiefly to the former. Succession 
of volcanic phenomena. The Eruptions, principally of intensely-heated aqueous vapour 
in torrents mingled with mineral matter, through fissures in the superficial crust of the 
globe, by which volcanos are formed, and in which their characteristic phenomena con- 
sist. jNlay be subaerial or submarine ; on the land or the bed of the ocean. The co- 
ruscations of lightning observed in volcanic eruptions. The circumstances of their pro- 
duction formerly shown to be analogous to those of the electric sparks from high-pressure 
steam issuing from a boiler, first described by Sir W. G. Armstrong. The source of 
electricity in that case proved by Dr. Faraday to be the friction of water and steam 
against other bodies. The entire process of excitation identical in both cases, and vol- 
canic lightning, therefore, probably the greatest example of frictional electricity in Na- 
ture. Accumulation of materials ejected in volcanic eruptions into a conical mountain, 
perforated by a kind of chimney, terminating at the summit in a funnel-shaped cavity 
called the crater. Intimate nature and immediate causes of eruptions. Characters and 
histoi-y of the currents of lava or molten rock, which rise up the chimney into the crater, 
and stream from it down the exterior of the cone, or flow through fissures in its flanks or 
near the base. Alternate production and repletion of the crater. Paroxysmal, conti- 
nued, and permanent eruptions. Various phases of volcanic action. Active and extinct 
volcanos. 
Lecture II. ['Nov. 19.) — Products of volcanic eruptions, aeriform, liquid, and solid ; the 
latter being of all dimensions, from blocks of enormous size to fine sand. Subsequent con- 
solidation of comminuted and pulverulent ejections, often exceeding the lavas in collective 
bulk, by means of water, into the substance called volcanic tufa or tuff. ^Many kinds of 
minerals ejected, others formed during or after eruptions. Lavas are of two kinds, the 
granular or stony, and the vitreous or glassy ; their varieties constituting an important 
class of the Igneous Rocks of geologists. The former may be arranged in general into 
three groups, basaltic, greystone, and trachytic lavas, all which are essentially aggregates 
of crystalline minerals, chiefly felspar, leucite, olivine, augite or pyroxene, and probably 
some others containing combined water; these minerals, chemically, being silicates of 
various earthy and alkaline bases , including oxide of iron, which also occurs separately 
as a mineral element of lavas in the form of titaniferous magnetic oxide of that metal. The 
peculiar nature of the fluidity of the Stony Lavas indicated by Dolomieu ; but their true hy- 
droplastic condition in the flowing state first observed and described, and the functions of 
water and its vapour in volcanic phenomena first adequately recognized, by Mr. G. Poulett 
Scrope. Probable agency in the production and effusion of lavas of the correlation and 
mutual convertibility of heat and mechanical force. The stony lavas apparently formed 
beneath the volcano by the disaggregation of previously existing lavas or other crystal- 
line rocks, effected by the expansive force, pressure, and solvent action of steam at a high 
temperature. This process a cooling one, and therefore the materials resulting from it 
not fused, but mingled with water and steam into a kind of ignited or incandescent mud, 
in which the fluid element, water or aqueous vapour, exists in a peculiar state of adhesion 
to and intervention between the surfaces of the mineral crystals and other solid particles. 
The Glassy Lavas (of three principal kinds, called obsidian, pitchstone, and pearlstone), 
formed originally by the opposite process of the conversion of mechanical force into heat, 
resulting in true melting or igneous fusion, at a still higher temperature than that at 
which the stony lavas become flowing. When steam is developed in them they become 
a kind of froth, which solidifies into pumice. Some plutonic or hypogeuous crystalline 
rocks, such as granite, also formed by a process of true fusion, in which water is an es- 
sential agent ; but these are of a distinct nature from that of lavas, and if they pass 
through the hydro])lastic condition are not raised in it to the surface of the earth, though 
they supply part of the materials for subsequent disintegration or fusion into lavas. All 
igneous rocks, however, graduate into each other in a manner variously related to their 
chemical constituents and composition, or those of the minerals which constitute them, 
and to the temperature at which they are formed and the time occupied in their solidifi- 
cation. Occasional sudden re-incandcscence of lava, with explosive discharge of steam, 
long after it has ceased to flow and become consolidated, being apparently the occur- 
rence, on a small scale, of the esscutial physical process of eruption. 
