Ch. XIII.] THE INCLINED POSITIONS OF STRATA. 269 



be sufficient for our purpose to remark, that the eruption of 

 volcanos is usually preceded by earthquakes, occasioned, as 

 is generally supposed, by the struggle of a highly expansive 

 power, such as intensely heated vapours, to overcome the 

 continuity of the incumbent rocks ; and these convulsions 

 are commonly terminated by the vapours finding vent, either 

 through previous craters, or through fissures produced by 

 their overcoming the resistance of the superimposed mass. 

 These fissures are sometimes simple chasms or ravines, of 

 greater or less extent, which are either superficial fractures, 

 or communicate by one or more channels with the volcanic 

 foci. 



Sometimes gaseous fluids only escape through these vents, 

 but more commonly they are accompanied by the discharge 

 of ashes and other incoherent substances, which, falling 

 around the fissure, form, by the accumulation of successive 

 layers, a conical mountain, the summit of which is an open 

 outlet or crater. These layers slope from the centre of the 

 mountain on all sides, as is evident when the ejecta have 

 periodically varied either in colour or composition, or when, 

 in consequence of its submarine origin, they alternate with 

 aqueous deposits. The same arrangement is also evident 

 when the volcano has poured out lava in a state of fusion, as 

 it also either partially or generally produces successive layers, 

 reposing on each other, or alternating with various igneous 

 products, according to the nature of the matter erupted. 

 Currents of lava are not, however, always disposed in this 

 regular manner : the convulsions to which these volcanic 

 masses are subject, often occasion fissures of various dimen- 

 sions, into which the lava flows, or is injected through chan- 

 nels connected with the central vent, and subsequently 

 consolidating, it has the form of dykes and veins, intersecting 

 the volcanic layers or strata in various directions. 



Whilst volcanos continue in action, the cone increases, its 

 inclined layers reposing on and gradually extending over 

 the fundamental rocks, which remain in their original 

 position. 



