ORIGIN AND STRUCTURE OF VOLCANIC CONES, jjj 



different origins, were there not existing intermediate ones which de- 

 monstrate that they are all varied modifications of one almost uniform 

 series of physical effects. 



Thus to one looking first at the vast volcanic cone of Cotopaxi, 

 almost perfect in form, and comparing it with the ring-like cavity of 

 Astroni in the Phlegrean field, it would be almost incomprehensible 

 to believe that these two extremes are the result of identical forces 

 acting much in the same manner and producing such widely different 

 effects. But in the latter district we have not to travel far to find 

 other vents that act as interpreters in explaining these variations of 

 forms. In the present pa2:)er it will be my endeavor to explain the 

 building up of what we Avill call a normal volcanic cone, and then 

 afterward to point out the extreme variations to which such a mass 

 is liable. 



Given a large volume of heated vapors and liquid rock that has 

 burst its way upward through the subjacent strata, in what way will 

 it manifest its presence, and what traces will it leave behind ? This 

 vapor does not seem to exist separately from the molten rock or lava 

 at any great depth, but as it approaches the surface the enormous 

 pressure is reduced, the water and other gaseous matter expand, 

 separate themselves into little bubbles scattered through the highly 

 heated liquid magma. These will collect, to a certain extent, and 

 from their lightness will float to the surface of the lava and there 

 burst. The vapor may have commenced to form at great depths, and 

 in its upward journey have become exceedingly bulky, so when it 

 reaches the surface it would escape with a loud explosion. If we 

 watch lava in the crater of a volcano in a quiescent state, such as 

 Vesuvius, we see these great bubbles, so to speak, continually form- 

 ing and bursting. As they burst, the surface of the vesicle is blown 

 up as soft, pasty fragments, to the height of many feet. These masses 

 appear black by day, but red-hot by night ; they may cool or not, 

 before falling ; if the latter, when they strike the ground, they adapt 

 themselves to the irregularities of the surface, and form, as it were, a 

 cast thereof. This condition is much exaggerated at the first outbreak 

 of an eruption ; the vast column of fragments often reaches an altitude 

 of two and three thousand feet. There the pieces ascending meet 

 those descending, and so there is a continual grinding going on be- 

 tween them ; the fine dust is taken by the wind and transported often 

 many miles, forming the so-called clouds of volcanic ash. The larger 

 fragments (or Icqnlli, as they are named) may again fall back into the 

 opening or around its edge, thus building up an annular bank. This 

 is really the foundation of the cone. 



If we speculate for a moment on the formation of such a heap, we 

 shall see that the first strata deposited will be horizontal, but some- 

 what thicker toward the axis of explosions. (See D, diagram.) This, 

 however, as the action continues, will begin to arrange itself in a 



