916 GEOGNOSY AND GEOLOGY. 
subterranean explosions. On the 28th of September, when the mouth had 
ceased to emit anything except sulphurous vapors, Prevost, in company with 
some fellow-voyagers, visited the island, and remained upon it for several 
hours. He ascertained the circumference to amount to 2000 feet, and the 
highest point of the crater to extend to an elevation of 200 feet above the 
level of the sea. The lake which filled the crater, and which stood at the 
same level with the ocean, was about about 180 feet in diameter. Pl. 50, fig. 
4, presents a view of the island, and fig.5 one of the inner crater. The island 
subsequently began to sink, standing at the level of the sea at the end of 
September, until at the beginning of December it had entirely disappeared. 
Quite similar circumstances attended the elevation of a small volcanic 
island, in 1811, near St. Michael, one of the Azores; it disappeared sub- 
sequently, so that now there is a depth of eighty fathoms and more over the 
summit. A figure of the island, at the time of its elevation, is presented in 
pl. 50, fig. 2. 
After the lightnings which accompany an eruption, and the subterranean 
explosions, have ceased to excite terror and apprehension in the hearts of the 
beholders, there sometimes arise luminous columns of fire, veiled in a black 
vapor. Nature appears then for a moment to be appeased and at rest: but 
new volcanic agencies commence which may be far more dangerous than any 
which have preceded. These are the mofettes, or gas springs, which, emitting 
noxious gases, such as carbonic acid, diffuse deadly poison throughout the 
entire region. While many of these soon disappear, others remain permanent 
for a long time, as the Grotto del Cane, near Naples (pl. 51, fig. 7), or else 
exist as acid springs in combination with water. 
Earthquakes generally announce an eruption; they are movements of the 
solid crust of the earth, whose cause or origin lies concealed within her 
bowels. A precise connexion between the two series of phenomena may 
not be strictly established, although such relation can in many cases be sub- 
stantiated. ‘The motions of the earth which constitute an earthquake are either 
horizontal and vertical, or rotatory : they are greatest in the centre of the field 
of influence, decreasing gradually to its borders. The extent of surface 
affected in a single system of earthquake is very various, and sometimes of 
great amount; in the earthquake of Lisbon it covered the half of Europe, 
and as far as the West Indies. Deep fissures are often formed by earth- 
quakes, such as those near Polistena in Calabria ( pl. 44, fig. 15), and some- 
times circular cavities, as in the plain of Rosarno (fig. 16), produced by the 
earthquake of 1783. 
The greater number of hot springs belong to volcanic exhibitions, as is 
well shown by their occurrence, in most cases, in voleanic regions. The 
most striking of these phenomena is, to be found in the case of the Geyser 
of Iceland (pl. 50, fig.1), a periodical sprmg, whose waters at a boiling 
heat are ejected to a considerable height in the air. The opening of the 
spring, or of the crater, lies on a hill consisting of silicious sinter, which the 
water had previously contained in the form of soluble silex. This action is 
shown in pl. 44, fig. 17. Iceland is especially rich in other volcanic 
phenomena of extraordinary grandeur. Pl. 45, fig. 10, is a chart repre- 
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