Considerations on Volcanoes. 361 



mass will be directly proportioned to the energy and duration of 

 the eruption. 



It is also plain that as these several circumstances may vary 

 during an eruption, so will variations take place in the appear- 

 ances and effects ; the chief circumstances subject to such changes 

 being the breadth of the aperture or the freedom of escape, and 

 the varying quantity of the superincumbent weight. Thus it is 

 that, during the eruptions themselves, the size of the aperture 

 enlarges by the abrasion of its sides, depending on the violence of 

 the expansive force, on the nature of the rocks traversed, and on 

 the depth from the surface of the focuskof expansion. Thus also 

 the consolidation of ejected lava within communicating fissures, 

 tends to increase the restraining power, and proportionally to 

 modify the results. 



A general law is hence deduced, namely, that the developement 

 of the volcanic action necessarily and universally tends to its own 

 extinction, by increasing the restraining forces. This follows 

 necessarily from the solidification which takes place within the 

 vent or vents, and from the quantity of ejected scoria which fall 

 back into the crater, and aid in choking it. 



Now when these powers have so acted as absolutely to consoli- 

 date and fill the passages, it may happen that a feebler point or less 

 resistance will occur at some place which had not formerly afforded 

 any passage. Thus an entirely new vent is formed ; and as it 

 is a probable inference that such weak parts will be found in the 

 line of former fissures produced by the previous actions of the 

 volcano, we are led to explain the fact of continued chains of vol- 

 canic apertures, such as are notedly found in America. 



As the volcanic aperture is a vent for the caloric contained 

 within the earth, when the eruption has not been such as to 

 carry off the whole then disengaged, or that the restraining forces 

 have suspended it, the consequence is, that the gradually increasing 

 expansion will break out into earthquakes ; and should a fissure 

 be formed, there will be what the author calls a paroxysmal erup- 

 tion. Thus he considers that some volcanoes have a double sys- 

 tem of operations, " productive both of paroxysmal and minor 

 eruptions ;" the first proceeding from interior foci, and having a 

 deep and wide crater, while the others occur in higher secondary 

 foci, throwing up cones within those craters, and tending to ob- 

 literate them. These are the moderate or tranquil eruptions and 

 actions, which proceed for a time till they are swallowed up by 

 one from the primary seat, of more intense power. Thus are 

 explained the well-known phenomena of Vesuvius, Teneriffe, 

 Bourbon, and other places, described by numerous observers. 



"We must here remark, before proceeding further, that the 

 author has given many explanatory drawings to illustrate the 

 minuter details into which he enters ; and as it is here in parti- 



