226 REACTION OF THE INTERIOR OF THE EARTH 



because,, notwithstanding the endeavour to generalise, it 

 still must always rest on imperfect inductions, one may 

 represent to oneself the bursting forth of igneously 

 fluid masses and solid substances, accompanied by 

 vapours and gases in various manners. Passing from 

 the more simple to the more complex phenomena, we 

 will specify, first, eruptions from fissures, not giving 

 rise to a series of detached cones, but pouring forth 

 substances in a state of tenacious semi-fluidity, forming 

 superposed volcanic masses ; secondly, eruptions through 

 cones of accumulation without surrounding escarpment, 

 but yet pouring forth streams of lava, as was the case 

 for five years at the devastation of the island of Lan- 

 cerote in the first half of the last century; thirdly, 

 craters of elevation with upheaved strata without any 

 central cone, sending forth streams of lava only on the 

 outer side of the escarpment, never from the interior, 

 which is soon closed by falling in; fourthly, closed 

 dome-shaped mountains, or cones of elevation open at 

 the summit, either surrounded by an 'at least partially 

 preserved encircling ridge, as at the Peak of Tenerifie, in 

 Fogo, and Eocca Monfina, or else quite without circum- 

 vallation and without any crater of elevation, as in 

 Iceland, ( 309 ) in the Cordilleras of Quito, and in the 

 middle part of Mexico. The open cones of elevation 

 of this fourth class maintain a permanent more or 

 less active communication, in indeterminate intervals 

 of time, between the heated interior of the earth and 

 the external atmosphere. Dome-shaped trachyte and 

 dolerite mountains with closed summits seem, according 

 to what I have been able to observe, to be more 

 numerous than open cones, either still active or extinct, 

 and much more numerous than volcanoes proper. Dome- 



