VOLCANIC PHENOMENA. 



ferent parts, each of which corresponds to a particular mode of 

 formation. The first gibbosity or hill is, in general, the effect of 

 elevation of the pre-existing soil, which may be of any kind or 

 nature. Afterwards, sooner or later a fissure is formed, which 

 produces either a crater of elevation or a dome of pasty matter, 

 as at Jorullo, clearly detached from the first hillock ; and, as a last 

 result, in the midst of one or the other a permanent chimney is 

 formed. Often the formation of the terminal cone then commences, 

 by the scoriaceous matters raised by the melted lava filling the 

 primitive conduit, which overflows the margin of the aperture, or 

 it is ejected into the air, from which it falls again around the centre 

 of eruption, accumulating in cones with a maximum slope of from 

 30 to 35. These loose scoriae melt on the side towards the inte- 

 rior of the chimney, which they narrow more and more by the suc- 

 cessive cornice-like projections they form, and in this way conceal 

 the true diameter of the crater. 



21. It is rare that these three kinds of formations are all found 

 in the same volcano ; bat we always find the gibbosity produced by 

 elevation, and one or the other of the secondary domes. At Tene- 

 riffe there is a broken dome which was upheaved in the middle of 

 a crater of elevation. At Vesuvius, from the constant solidity of 

 the base, and other circumstances, we may infer the existence of a 

 central nucleus, produced in the same way as a dome, in the year 

 79, afterwards enveloped in loose materials, and bearing on its 

 summit a true cone of scoriaB. At Etna (Jig. 198) we clearly 



Fig. 198. View and profile of Etna, and the surrounding country. 



distinguish the primitive hill or gibbosity, showing sheets or 

 coats of ancient upheaved lavas, on the middle of the slightly- 

 arched surface, which all this part of the island presents ; it is 

 terminated by an almost level surface, the Piano del Lago, in 

 the midst of which rises the terminal cone of scoriae, regularly cir- 

 cumscribed on all sides, and clearly separated from the base on 

 which it was formed. On the slopes are small cones of eruption, 

 formed here and there, at different times, which have since contri- 

 buted to the swelling up of the whole of the surrounding land. 



22. It is clear, that the cones of scoriae constructed in the man- 

 ner just mentioned, at the bottom of volcanic gulfs, cannot be very 

 solid: they often change their form at every eruption. Sometimes 

 the edifice rises more and more ; sometimes, on the contrary, it 



20. Are volcans always characterized by the same kind of formations? 



21. Do we always find in one volcano all the kinds of formation? What 

 one is always found ? 



22. What are the characters of cones of sconce found at the bottom of 

 Wcanic gulfs ? 



