610 GEOLOGY, 



the cinders are still plastic when they fall, and weld themselves together 

 and hold their places even on \Qxy steep slopes, but usually they have 

 already hardened before they reach the surface. 



Sudordinate cones. — Small or temporary vents formed as offshoots 

 from the main vents often give rise to secondary or ''parasitic" cones. 

 These are sometimes numerous, as in the case of Etna, and they may 

 be so important that the mountain becomes a compound cone. A still 

 more subordinate variety consists of ''spatter-cones" formed by small 

 mildly explosive vents that spatter forth httle dabs of lava which form 

 chimneys, or cones, and sometimes completely curved domes over 

 vents (Figs. 466 and 467). Spatter-cones often arise from the lava- 

 flows themselves. 



Composite cones. — From most existing volcanoes there issue both 

 lava-flows and fragmental ejecta, and the resulting cones are composite 

 in material. The lava more frequently breaks through the side of the 

 cone than overflows its summit, and this gives rise to irregularities of 

 form and structure. The cones are also subject to partial destruction 

 both by the outbursts of lava and by the explosions, and perhaps also 

 by migration of the vents. As a result, many volcanic regions show 

 old, partially destroyed craters, together with new and more perfect 

 ones, and the history of volcanic action in a region may often be read 

 in the succession of cone formations. 



The form of the cone, when composed chiefly of lava, is also 

 affected by the mass of the outflow and by its fluidity. The larger the 

 outflow at a given time, other things being equal, the wider it dis- 

 tributes itself and the flatter is the cone. As a rule, the basic lavas 

 are more fluid than the acidic, and the cones of basic lavas are flatter 

 than the cones of acidic lavas. 



Extra- cone distribution. — In violent eruptions, the steain, accom- 

 panied with much ash, is shot up to great heights, often rolling out- 

 wards in cumulus or cauliflower-like forms (Fig. 458). In the more 

 violent explosions these colimins are projected several miles. In the 

 phenomenal case of Krakatoa the projection was estimated at seven- 

 teen miles. The steam, b}^ reason of its great expansion and its contact 

 with the colder regions of the upper air, is quickly condensed, and 

 prodigious floods of rain frequently accompany the eruption. This rain, 

 carrying down a portion of the ash and gathering up much that had 

 previously fallen, gives rise to mud-flows, which in some cases consti- 



