116 Volcanos. 



by the injection and consolidation of lava, which open down- 

 wards. Others which increase in width towards their upper 

 extremity, will remain open, and effectually weaken that part 

 of the crust of rocks across which they are broken. Subse- 

 quent expansions, taking effect most powerfully on these weak 

 points, will widen and extend these fissures until some one is 

 sufficiently deep and broad to permit the lav^ in the lower parts 

 of the fissure to rise by its intumescence into communication 

 with the atmosphere on one or more points at the upper extrem- 

 ity, thus producing a volcanic vent or vents. 



" In most instances, the fissure must be narrow, irregular, and 

 intricate, and the distance great from the external surface to the 

 focus of ebullition. The intumescence will be proportionately 

 slow, and the repressive force, consisting of the weight of the 

 rising column of lava, and the accumulation of fragments bro- 

 ken from the sides of the fissure, may stifle the ebullition be- 

 fore the lava has reached the lips of the orifice. Such may be 

 called an abortive eruption, vapor alone escaping outwardly before 

 the fissure is closed. The author attributes the clouds of smoke 

 or vapor, and projections of fragments that have been dis- 

 charged during violent earthquakes from crevices in the soil 

 to a subterranean effervescence of this nature.* But where the 

 width of the fissure, and other circumstances permit it, the la- 

 va reaches the mouth of the vent, and a regular volcanic erup- 

 tion occurs. 



" The author proceeds to examine the laws which regulate 

 the developement of the eruptive force. This is opposed by the 

 repressive force, consisting of, 



" 1. The supported eolumn of liquid lava; 



" 2. The reaction of the vapor generated from the impedi- 

 ments to its expansion ; 



" 3. The external pressure on the surface of the intumescent 

 lava. 



" Of these elements, the last is the most subject to variation, 

 particularly from changes, 1. In the dimensions of the vent; and 

 2d. In the quantity and weight of matter pressing on the surface 

 of the lava within the vent. 1. The violent rise and explosive 



* It appears that the aqueous vapor emitted from fissures in the 

 surface soil, during earthquakes is in general very great ; since Ferra- 

 ra mentions that extraordinary storms of rain immediately follow the 

 occurrence of these phenomena. In the violent earthquake which af- 

 fected the whole northern coast of Sicily in 1823, a remarkable dense 

 black cloud collected over the district affected, and shortly was conden- 

 sed into terriffc deluges of rain. The same thing happened during 

 the great earthquake of Catania in 1693, and that of Calabria in 1783. 



