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 Geology. 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. May 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 tbose 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 lightyiing, 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 

 history 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 tufl". Many kinds of 

 minerals ejected, others formed dui-ing 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 fi'om 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 

 wliich the stony lavas become flowing. When steam is developed in them they become 

 a kind of froth, which solidifies into pumice. Some plutonic or hypogenous 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 

 llirough the hydroidastic condition are not raised in it to the surface of the earth, though 

 t liey 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 

 clieinical 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 rc-incandescence of lava, with explosive discharge of steam, 

 long after it lias ceased to flow and become consolidated, being apparently the occur- 

 rence, on a small scale, of the essential physical process of eruption. 



