^66 Observations on. Volcanoes and their Lava. 



white quart2y rock; it is found sometimes of calcareous 

 rock. 



The most probable idea that can be formed in regard to 

 the origin of these fragments is, that they have been carried 

 from the borders of the strata through which the lava, that 

 comes from great depths, has opened for itself a passage. 

 These fragments arc carried to the surface of the lava as far 

 as the bottom of the chimney of the crater, v^hence they 

 have been thrown out by explosions, mixed with fragments 

 separated, or rather torn, from the lava; for it is not by the 

 lava that they have been brought forth to view, but by ex- 

 plosions. 



Some of these fragments of natural rocks have not been 

 attacked by the fire ; others have more or less ; which de- 

 pends, no doubt, on the place which thev occupied in the 

 volcano, and on the time which they remained in it. The 

 most of the latter have retained at their surface a crust of 

 lava, and this crust contains substances which are not the 

 same as that of the fragment it covers. 



On Vesuvius the strata pierced by eruptions are lower 

 than the surface of the soil; in Auvergne and several places 

 of Germany they are above ; for this reason there are seen 

 there in their place schists or granites, which the eruptions 

 have broken to form for themselves a passage. 



No volcano rests on natural strata ; thev sometimes show 

 themselves on the exterior ; but they have been opened by 

 eruptions, and their edges have remained in their place. 



The focus of no volcano exists or has existed in the cone 

 which appears above the surface of the ground. They have 

 been raised by eruptions, which, proceeding from great 

 depths, have thrown them np through the upper strata. 

 When it is said, therefore, that the volcanic mountains of 

 Auvergne rest on granite, this is a mistake, and an incor- 

 rect expression has been used by those who have not form- 

 ed a just idea of the phrenomenon. Lava may have flowed 

 upon granite or any other rock, and rested upon it ; but 

 this is never the case with the volcano itself: its bases are 

 below all the rocks visible. 



It is from the bosom even of the lava, when in a state of 

 fusion in the interior of the volcano, that all the explosions 

 proceed. In that state of fusion they contain all the mat*- 

 ters which produce fermentations, and the disengagement 

 of expansible fluids. 



I have been enabled tf) ascertain this on Vesuvius as far 

 as was possible. The continual noise which was heard 

 through the two interior mouths of the crater which I had 



before 



