256 



VOLCANOES. 



of subterranean thunder were sometimes heard which resembled at- 

 mospheric thunder, but were inconceivably louder and more appal- 

 ling : ihey were followed by earthquakes. Humboldt also mentions 

 the frequency of subieranean thunder in some districts bordering the 

 Andes. 



In volcanic phenomena, we observe a cause in present activity, 

 that can overthrow mountains, form new islands, and raise up the 

 bed of the ocean : hence the geologist may infer, that the same 

 cause, acting with greater intensity and more extensively, has been 

 the agent employed by the Author of nature, to elevate new and 

 submerge ancient continents, and to change and renovate the surface 

 of the globe. We are indeed acquainted with no other natural agent 

 that can have effected the mighty changes w^iich the crust of our 

 planet has undergone. The products of volcanoes, particularly of 

 ancient ones, are analogous in their composition and internal struc- 

 ture to the oldest rocks of granite, sienite and porphyry, and indi- 

 cate not obscurely, the mode in which these rocks were formed : 

 hence the study of volcanoes and volcanic rocks, is an important 

 branch of the science of geology. Werner and his disciples, how- 

 ever, held that volcanoes were produced merely by the ignition of 

 beds of coal, in the secondary strata. 



Volcanoes are openings made in the earth's surface by internal 

 fires ; they, regularly, or at intervals, throw out smoke, vapour, flame, 

 large stones, sand and melted stone called lava. Some volcanoes 

 throw out torrents of mud and boiling water. Volcanoes, most fre- 

 quently, exist in the vicinity of the sea or large lakes, and also break 

 out from unfathomable depths below the surface of the ocean, and 

 form new islands and reefs of rock. When a volcano breaks out in , 

 a new situation, it is preceded by violent earthquakes, the heated sur- 

 face of the ground frequently swells and heaves up, until a fissure or 

 rent is formed sometimes of vast extent. Through this opening masses 

 of rock, with flame, smoke and lava, are thrown out, choke up part of 

 the passage, and confine the eruption to one aperture or more, round 

 which conical hills or mountains are formed. The concavity, in the 

 centre, is called the crater. The indications of an approaching erup- 

 tion from a dormant volcano, are an increase of smoke from the 

 summit, which sometimes rises to a vast height, branching in the 

 form of a pine tree. Tremendous explosions, like the firing of ar- 

 tillery, commence after the increase of smoke, and are succeeded 

 by red coloured flames, and showers of stones. At length, the lava 

 flows out, from the top of the crater, or breaks through the sides of 

 the mountain, and covers the neighbouring f>lains with melted matter, 

 which, becoming consolidated, forms a stony mass, often not less 

 than some hundred square miles in extent, and several yards in thick- 

 ness. The eruption of lava has been known to continue, several 

 months. Intensely black clouds, composed of a kind of dark colour- 

 ed sand or powder, improperly called ashes, are thrown out of the 



