ON ITS EXTERIOK. VOLCANOES. 259 



slide down a part of the cone, and are seen, at night, 

 shining afar in the darkness : it was this probably which 

 gave occasion to La Condamine's erroneous belief of 

 "burning sulphur and asphalte teing poured forth." 

 The stones are thrown up individually and successively, 

 so that some are falling back while others are rising. By 

 exact measurement by time, the visible space of fall 

 (reckoning therefore to the margin of the crater) was 

 determined in the mean to be only 785 feet. At Etna, 

 the stones thrown up reach, according to the measure- 

 ments of Sartorius von Waltershausen and the astronomer 

 Dr. Christian Peters, a height of 2660 feet above the 

 crater. Gremellaro's estimations, during the Etna erup- 

 tion of 1832, were even three times as high ! The 

 black ejected ashes form, on the slopes of Sangay and 

 for twelve miles around, beds 300 or 400 feet thick. 

 The colour of the ashes and rapilli gives to the upper 

 portion of the volcano a peculiarly dismal and fearful 

 aspect. Before closing this paragraph, I would once 

 again call the reader's attention to the size of this great 

 volcano (six times the height of Stromboli), because it 

 militates strongly against the belief of the lower volcanoes 

 having always the most frequent eruptions. 



Even more important than the form and height of 

 volcanoes is their grouping, because it leads us to the 

 great geological phenomenon of elevation over fissures. 

 Such groups of volcanoes, whether they have been 

 elevated, according to Leopold von Buch, in lines, or 

 simultaneously around a central volcano, indicate the 

 parts of the earth's crust in which (whether it may 

 have been from the lesser thickness of the rocky strata, 

 or from their nature, or from their original fissuring) 

 the tendency of the molten interior to break forth has 



