VOLCANIC PHENOMENA. LAVA-CURRENTS. M7 



country, and an immense volume of melted matter escaped from 

 them. This immediately spread over the soil, covering eighty 

 square leagues, filling up all depressions, and forming a vast lake 

 of fire of considerable depth. 



33. But this is not always the case ; the current often forms 01 

 more or less inclined slopes, arid the lava forms true currents on 

 their surface, of greater or less length, a part of which adheres to 

 the land in consequence of cooling, and in evidence of its passage. 

 After its exit from the bosom of the earth, the melted matter soon 

 cools cm the outside, solidifies, wrinkling and cracking in every 

 direction, and thus acquires a crust, ordinarily porous, the thicknes.-j 

 of which becomes more or less considerable. This crust prevents 

 the liquid or paste it envelopes from spreading, and confines the 

 current to a certain thickness ; also, from its slight faculty of con- 

 ducting heat it prevents the interior lava from cooling, which, from 

 this cause, goes on very slowly. Lavas have in fact remained 

 liquid or pasty, and preserved a high temperature for a very con- 

 siderable time ; some are cited as still running on very gentle 

 slopes, ten years after their ejection, and others which gave off 

 vapour twenty-six years after their exit from the bosorn of the earth. 



34. If after the external cooling the volcanic spring continues 

 to furnish melted lava, the current takes place in a kind of con- 

 solidated sack which is formed ; a sack which then strives, as it 

 were, in all directions, is broken and mended successively; this 

 causes the twisting and various irregularities in the current of 

 lava. When the source is stopped, the matter which escaped from 

 it does not continue to flow the less in the sack enclosing it, but the 

 latter successively flattens, and the middle is effaced, leaving a 

 more or less elevated roll or ridge on the margins. This is first 

 seen at the upper part of the current, then successively to a poi.nl 

 where the liquid matter, becoming 



more and more viscid, has not suffi- 

 cient force to drag after it the solid 

 parts formed, to break or push them 

 forwards. The lava then stops at 

 the bottom of the sack, terminating Fig> 200. Lava. current arrested 

 in a club-like mass (fig. 200). The on a sl P e - 



form, direction, and extent of these lava-currents vary according 

 to circumstances, such as the degree of inclination of the mountain 

 sides, and the nature of the lava itself. Some volcanic products 

 are so pasty they cannot run, but remain over the aperture, as 

 occurs with certain trachytes, which then form more or less elevated 

 domes. Others, such as various obsidians, which seem to cool 

 and harden quickly, are sometimes arrested in form of great tears, 



33. Do lava currents cool rapidly under all circumstances ? 



34. Is the form, direction, and extent of lava-currents always the same f 



