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PHYSICAL GEOLOGY 



e of slope of volcanoes. A cone com- 

 posed of ash will be steep, as much as 30 , while that of 

 lava may not be more than 9 , as in the Hawaiian Islands. 



angle of repose, which may be as great as 30 or 40 for coarse ash or 

 cinders (Fig. 305 A). The slope is more gentle, however, at the base 

 of the cone, since the dust is carried farther from the summit than the 



coarser material and 

 is washed farther still 

 by rain and rills. If 

 the cone is of lava, 

 its slope will depend 

 upon the fluidity of 

 the lava. Volcanic 

 cones built of basic 

 lava usually have 

 broad, flattened domes (Fig. 305 B), since such lava cools at a low 

 temperature and consequently may remain liquid for a considerable 

 time and flow long distances before solidifying. The Hawaiian and 

 Icelandic volcanoes are examples. If stiff, viscous lava is dis- 

 charged, the slope may be very steep (Fig. 307). Cones made of a 

 combination of lava and ash are more common than any others and 

 are usually steep-sided. The vol- 

 cano Fuji, so often pictured by 

 the Japanese aitists, is of this 

 type, as are many of the highest 

 volcanoes of the world. 



A volcanic cone is seldom sym- 

 metrical, since if it has been long 

 in existence it has suffered many 

 changes (Fig. 306). The irregular 

 outline of Vesuvius, as has been 

 seen, is the result of the blowing 

 off of the greater part of an 

 earlier crater, so that the present 

 cone is partially surrounded by 

 Mt. Somma, a remnant of the 

 ancient crater. The slopes of 

 Etna are roughened by scores of 

 parasitic cones. The volcano 

 Colima, in Mexico (Fig. 308), 

 would be beautifully symmetrical were it not for a cone formed on 

 the flanks of the mountain in 1869. This secondary cone is crater- 

 It ss, showing that near the close of the eruption the lava was so 



Fig. 306. — Outlines of volcanic cones: 

 J, a cone formed of ash; B, a cone from 

 which the top was blown by a great explo- 

 sion; C, a caldera formed by faulting. 



