746 DYNAMICAL GEOLOGY. 



The moisture must hence continue to be at the high temperature of 

 fusion or semif usion until the final escape. 



Further : Through the action described, the column of liquid lavas 

 is lengthened upward ultimately some hundreds of feet. This ex- 

 poses a far larger surface and amount of liquid lava to the waters of 

 the island, and in a more accessible condition. The action becomes 

 consequently more rapid and vivid, the escaping vapors more abundant, 

 the floodings of the crater's bottom more frequent, with greatly en- 

 hanced risks of catastrophe from any sudden supplies of water. Fi- 

 nally, owing to the augmented pressure of the lava-column, and the 

 pressure of the vapors seeking escape, a break occurs, and a discharge. 



Prevost compared the rise of the lavas in a volcano and the over- 

 flow to the frothing over of a molasses cask at the bung-hole ; and the 

 illustration is instructive. 



Rev. Titus Coan, to whom science owes much for his observations 

 on the Hawaiian volcanoes, states that during the months previous to 

 the violent eruption of 1868, when the summit-crater and Kilauea 

 were in simultaneous action, and the island was convulsed with earth- 

 quakes, there had been unusual rains, and he attributes to this fact the 

 unusual violence of the eruption ; and it has been observed elsewhere 

 that rainy seasons are most favorable to eruptions. 



In volcanoes with more viscid lavas, like Vesuvius, the course, as 

 the facts described on page 733 show, is in general the same ; but the 

 resistance to the escaping vapors, due to the viscidity of the lavas, is 

 vastly greater ; and, consequently, the liability to fracture when an 

 eruption is near at hand, — so giving access to the sea or some subter- 

 ranean fresh-water streams — makes such volcanoes more liable to 

 catastrophic eruptions. It also gives the cones steeper sides, and, 

 therefore, less strength for resisting fracture. 



What proportion the moisture from the deep source of the lavas of a 

 volcano bears, in volcanic action, to that from superficial sources it is 

 not possible to say. The former may be a constant force causing up- 

 ward movement, and a supply of heat and lava, and so act against the 

 cooling influence of superficial waters ; the latter, a cause of the daily 

 and yearly variations of condition, and of the eruptions. 



Deville has presented the view that hydrogen from the dissociated elements of water, 

 is an essential agent in volcanic action ; and essentially this view was held by Delanoiie, 

 Angelot, and Elie de Beaumont. It is not yet sustained by facts from the emanations 

 which come direct from the volcanic focus, nor by any peculiarities in the lavas, such 

 as would arise from the oxj'gen that should in that case have been free to enter into 

 new combinations. 



(2.) Non-volcanic Igneous Ejections. — The wide extent of the out- 

 flow of igneous rocks in some non-volcanic regions appear to indicate 

 that the areas of fusion beneath the earth's surface have often had 



