338 Analysis of Scientific Books and Memoirs. 



and also often occasions the crystals composing the lateral parts of the 

 vein to be more comminuted than those in the centre. The matter filling 

 these veins is immediately consolidated both by loss of temperature and 

 pressure, and the fractured rocks are thus repaired and strengthened. An 

 interval of tranquillity will then succeed, until a similar expansion occurs. 

 It is thus that the overlying rocks which form the surface of the globe 

 must be progressively elevated more and more, by the successive dilata- 

 tions of the inferior lava-bed, unless some avenues are opened within a 

 limited distance, for the more tranquil escape of the subterranean ca- 

 loric. 



But the formation of such apertures (volcanic spiracles) must sooner or 

 later result from the continuance of this process ; for, at every crisis of 

 expansion, those cracks only are repaired by the injection and consolida- 

 tion of lava, which open downwards. Others which increase in width to- 

 wards their upper extremity, will remain open, and effectually weaken 

 that part of the crust of rocks across which they are broken. Subsequent 

 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 lava in the lower parts of the fissure to rise by its intumes- 

 cence into communication with the atmosphere on one or more points at 

 the upper extremity, 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 ebul- 

 lition. The intumescence will be proportionately slow, and the repres- 

 sive force, consisting of the weight of the rising column of lava, and the 

 accumulation of fragments broken from the sides of the fissure, may stifle 

 the ebullition before the lava has reached the lips of the orifice. Such 

 may be called an abortive eruption, vapour alone escaping outwardly be- 

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

 vapour, and projections of fragments that have been discharged during vi- 

 olent earthquakes from crevices in the soil to a subterranean effervescence 

 of this nature.* But where the width of the fissure, and other circum- 

 stances permit it, the lava reaches the mouth of the vent, and a regular 

 volcanic eruption occurs. 



The author proceeds to examine the laws which regulate the develope- 

 ment of the eruptive force. This is opposed by the repressive force, con- 

 sisting of, 



1. The supported column of liquid lava ; 



2. The reaction of the vapour generated from the impediments to its 

 expansion ; 



" It appears that the aqueous vapour emitted from fissures in the surface soil 

 during earthquakes is in general very great ; since Ferrara mentions that extraor- 

 dinary storms of rain immediately follow the occurrence of these phenomena. In 

 the violent earthquake which affected the whole northern coast of Sicily in 1823, a 

 remarkable dense black cloud collected over the district affected, and shortly was ««w 

 densed into terrific deluges of rain. The S3tne thing happened during the g»eat 

 earthquake of Catania in 1 693, and that of Calabria in 1 783. 



