ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 337 



but eye witnesses can form any idea. Sartorius Von Waltershau- 

 sen thus describes a volcanic eruption principally in relation to the 

 various products formed by it : "During an eruption the melted 

 " matter ascends from the deeper lying regions of the earth into 

 " the volcano where it serves both for the formation of volcanic 

 ash and of lava. Steam prodigiously compressed tries, where it 

 " can, to break through the column of lava to the atmosphere. 

 "This escape of steam is the principal cause of that subterranean 

 " noise known as volcanic thunder. A continual struggle takes 

 " place between the elastic fluid, the fused mass and the solid 

 " walls of the volcanic canal, which struggle lasts so long as the 

 " development of steam in the latter continues. During this 

 i% violent ascent of the enormous steam bubbles, which burst on 

 " reaching the surface of the lava reservoir, pieces of lava already 

 " cooled or still fluid are violently torn off from the latter and 

 " thrown high up in the air out of the crater. When the eruption 

 " is at its height, millions of these pieces, mostly red hot, from the 

 " size of mere miscroscopic particles to those with a diameter of 

 " one or more yards, fill the air above the crater, rising in myriads 

 " with each explosion, and falling again in perpetually changing 

 "motion. When the intervals between each explosion are short, 

 " as is the case with all violent eruptions, it frequently happens, 

 u that during a lapse of about 20 seconds, which time the glowing 

 " stones frequently take to complete their passage through the air, 

 " six to ten new explosions take place. It is evident that an uninter- 

 " rupted volley and shower of stones mixed with the dense smoke of 

 " finer particles will thus be sustained, and this it is, which, partly 

 " glowing itself, and partly lighted up by the glow of the melted lava, 

 " in the crater, resembles a permanent flame. The fragments of 

 "lava thus thrown out of the crater differ from each other in size, 

 " in external form, (which is frequently determined by the tempe- 

 "rature at which they were formed,) and in chemical composition. 

 " Blocks have been observed measuring 4 to 5 metres each way, 

 " smaller ones about the size of a cubic yard occur frequently, 

 * while from this size there are innumerable gradations down to 

 " the finest dust. During an eruption, gravitation and the force 

 " of the wind effect a separation of the fragments according to their 

 " sizes. The largest of them fall back into or close around the 

 " crater, the small pieces are thrown further, while the finer par- 

 " tides are borne off by the wind and gradually deposited from it, 

 " the coarser particles first, and ultimately the finest dust, which is 



