VOLCANIC PHENOMENA. CRATERS. 107 



Let us add that islands which rise to the surface of seas do not always 

 remain. Many of them disappear after a longer or shorter period, either 

 by being washed down by the waves, as is supposed to have been the case 

 with the island of Julia, or by their mass sinking into an abyss formed be- 

 neath them ; the last circumstance doubtlessly happened to an island which 

 was elevated in 1719, near Saint-Michael (Azores), and disappeared in 

 1723, leaving in its place a depth of seventy fathoms. In the same region 

 there was an island in 1638, where there is now a bottomless abyss. 



1 4. Crater of upheaval, or elevation. The first effect of an 

 eruption is to burst, by its violence, the crust of the earth in the 

 direction which matters pent up in the interior have taken to 

 escape. The ground, no matter of what nature, is at first raised 

 to a more or less considerable extent, or arched like a bell, and 

 often cracked in every direction ; at once, the explosion occurring,, 

 as if by the action of a formidable powder-blast, an opening i s 

 made in the form of a fennel, through which often escape gaseous 

 and other matters which caused the event. It is to these initiatory 

 openings, which may be made anywhere, to which the name of 

 crater of elevafion has been given, from the necessity of distin- 

 guishing them from all that may subsequently occur in the series 

 of volcanic phenomena. The hillock itself which is produced on 

 the soil, by the first effect, is called the cone of elevation, to distin- 

 guish it from analogous hillocks which are often formed also by 

 the accumulation of incoherent materials ejected from the volcano. 



15. Character of these openings. What characterizes craters 

 of elevation, and enables us to recognise them in places where 

 there is no account of an eruption, is, the disposition or arrange- 

 ment of the upheaved strata, being very different from what is 

 everywhere else observed. These beds are here found inclined all 



round the axis of the cone, as in the 

 section (Jig. 188), rising more and 

 more from the base to the summit, 



and presenting their abrupt escarp- 



Fiff. 188. Disposition of strata * * r c if 



around a crater of elevation. ments towards the Ulterior of the 



cavity. Monte-Nuovo is an exam- 

 ple in miniature : the mountain was formed by elevation, hollowed at 

 its summit by ejecting gases and incandescent matters ; and the 

 cavity, which can be examined now, has around it, at an inclination 

 of thirty degrees, strata of different formations, which in all the 

 rest of Campa'nia are horizontal. The semicircle of the somma 

 presents the same characters in the inclined tables of amphige'nic 

 porphyries, and analogous circumstances exist in many other 

 localities. 



1(5. Another character, not less important, and especially useful 

 when the upheaved matters are not divided into beds, is furnished 



14. What is a crater of elevation ? What is a cone of elevation ? 



15. How are craters of elevation characterized ? 



30* 



