ON ITS EXTERIOR. VOLCANOES. 223 



quarrying (not decomposed rolled pumice-stone brought 

 by water), appearing to be quite independent of the 

 mountains, which are seen rising only at a considerable 

 distance. Why may we not suppose that, in the pro- 

 gressive coolingdown of the heat-radiating outer strata 

 of the earth, before either isolated mountains or moun- 

 tain chains were yet upheaved, the surface may have 

 been variously split up and fissured ? and why may 

 not these fissures have extruded igneously fluid masses 

 which have hardened into volcanic and other rocks 

 (trachytes, dolerites, melaphyres, pearlstone, obsidian, 

 and pumice) ? Part of these, originally in horizontal 

 beds (trachytes or dolerites which had burst forth in 

 a tenacious semi-fluid state, as from earth-springs), ( 305 ) 

 may, at the later upheaval of volcanic cone- and dome- 

 shaped mountains, have been thrown into a broken and 

 precipitous state, not seen in the later lavas which have 

 originated in burning mountains or volcanoes proper. 

 Thus, to recall first a well-known European example, 

 in the Val del Bove on Etna (a hollow which cuts deep 

 into the interior of the mountain), the angle of " fall " 

 of the lava strata, alternating very regularly with rolled 

 masses, is from 25 to 30 ; whereas that of the streams 

 of lava which cover the surface of Etna, and which have 

 flowed since its upheaval as a mountain, have, on a 

 mean of thirty streams, according to Elie de Beau- 

 mont's exact determinations, only an angle of descent of 

 from 3 to 5. Such differences point to the existence 

 of very ancient volcanic formations which had burst 

 forth from fissures before the elevation of the existing 

 volcano. Classical antiquity also presents us with an 

 example of a remarkable phenomenon of this class, 



