780 GEOLOGY. 



and during that space of time its operations have been unceasing, though very rarely 

 violent. Lava never flows over the top of the crater, but it is occasionally discharged 

 through fissures into the sea, killing the fish ; and Stromboli has been observed to be 



more active in stormy than 

 in fair weather, likewise in 

 winter than in summer 

 a fact explained by the 

 isle of Paima. different degrees of pressure 



exerted by the atmosphere upon the lava at different times. The internal force pre 

 dominates when the atmospheric pressure is lessened. In most cases, however, the 

 operation of active volcanoes is paroxysmal, an interval of repose following any marked 

 display of energy, the time of dormancy varying from a few months to many centuries. 

 With a very slight interruption, Vesuvius slumbered from the year 1306 to 1631, or for 

 three and a quarter centuries ; but, in the island of Ischia, seventeen centuries have been 

 known to intervene between two eruptions. Hence it follows, that as the civilised 

 world has only been acquainted with the western continent for a comparatively brief 

 period, while a knowledge of its phenomena is still more recent, some of those Andean 

 peaks, Chimbora9O in Quito, Tacoza in Peru, and Nerado de Toluco in Mexico, which 

 are regarded as extinct volcanoes, may yet break forth, and show themselves to belong 

 to the intermittent class. 



Volcanic formations consist principally of lava, or melted rock-matter, which is either 

 upheaved by immense mechanical pressure through the hollow interior, so as to flow from 

 the top of the volcano in eruption ; or, as is usually the case, it makes for itself lateral 

 passages on the flanks of the mountain, and overspreads the adjacent districts, sometimes 

 to a considerable distance, filling up valleys, dividing streams and diverting their course, 

 and elevating plains by adding broad and thick expanses of material to them. Lava is 

 chiefly composed of the two minerals, felspar and augite, with titaniferous iron. When 

 the felspar predominates, light-coloured lavas are the result, called felspathic or trachytic; 

 but when the augite is in excess, dark varieties, augitic or basaltic lavas are produced. 

 Other simple minerals occur, upwards of a hundred species having been noticed in the 

 single instance of Vesuvius, but entering in very inconsiderable proportions into its pro 

 ducts. Besides varying in chemical composition, the character of lava is greatly modified 

 by the circumstances under which it is cooled down, compact rock resulting from cooling 

 under pressure, but a porous, fibrous, and light material being produced when cooled with 

 the sole weight of the atmosphere upon it. If we suppose a current of matter of the con 

 sistency of honey, we shall have a tolerably correct picture of a stream of lava, which, as it 

 moves over a district, is not thinned but walled at the sides, and, a crust soon forming upon 

 the surface, tends still more to prevent it spreading out laterally. Hence it is sometimes 

 possible, when the ground favours the attempt, to deflect one of these fiery floods from 

 its course by attacking the sides and removing the crust, which has succeeded in turning 

 it away from towns that were threatened by it. In one instance the inhabitants of Catania 

 assailed a lava current in this way to divert it from their city, when the people of another 

 locality, who were placed in jeopardy, took up arms to resist the operation. Lava rapidly 

 cools externally; but the crust formed being a very bad conductor of heat, the interior of the 

 mass remains hot, semi-fluid, and moves on after the surface has solidified. The stream 

 thrown out of Etna in 1819 was in motion at the rate of a yard per day, nine months 

 after the eruption ; and another from the same mountain is stated not to have become 

 completely consolidated and at rest ten years after its emission. Dr. Daubeny states, 

 speaking of temperature, that some days after ejection lava has raised the thermometer 

 from 59 to 95 at the distance of twelve feet, while, three feet off, the heat greatly 



