THE CRUST OF THE EARTH. 



matter, which appears to have been the result of former ejections. 

 The middle is often formed of a lake of mud, the surface of 

 which is more or less consolidated. In certain countries these 

 mounds are found permanently dried, all disengagement of gas., 

 water, and earth having altogether ceased, but it sometimes 

 happens that the samg phenomena after long cessation are 

 renewed with violence. 



158. Fumarolles and Geysers are the names given to eruptions 

 of steam or boiling water issuing from crevices in volcanic dis- 

 tricts, remarkable examples of which are presented in the country 

 surrounding the celebrated volcano of Hecla, in Iceland. Erup- 

 tions of hot steam are projected from the crevices of the soil 

 in the form of white columns, rising to heights of from 30 to 

 60 feet, and often with noise similar to that with which high 

 pressure steam issues from the safety-valve of a boiler. Such 

 phenomena are manifested on a considerable scale in Tuscany, in 

 the neighbourhood of Monte Cerboli, Castel Nuovo, and Monte 

 Rotondo, and are generally disposed in a single line of from 20 to 

 25 miles in length. 



These jets of vapour in all cases include chemical agents, which 

 attack the rocks with which they come in contact; thus the 

 vapour ejected from Vesuvius includes hydrochloric acid, that of 

 the Solfatara, of Pozzuolt, includes sulphurous acid, and that of 

 Tuscany, boric acid. 



159. The Geysers are volcanic eruptions of boiling water, some 

 continued, others intermitting, which prevail in immense numbers 

 in Iceland. One of these hot springs is mentioned which, from 

 half-hour to half-hour, projects a column of boiling water 18 feet 

 in diameter to a height of 150 feet (fig. 71). 



The water thus ejected contains a certain proportion of silica, 

 which is deposited in a state of hydrate upon all the surround- 

 ing bodies, and forms sometimes mounds of considerable extent, 

 at the summit of which is an opening, from which the liquid 

 issues. 



Besides silica, the water of the geysers also contains, in a small 

 proportion, the carbonates or sulphates of soda, ammonia, potash, 

 and magnesia, besides a minute proportion of carbonic acid. 



160. Calcareous as well as volcanic countries present vast 

 depressions of the ground analogous to craters, but instead of 

 being nearly circular like those of volcanoes, they are most fre- 

 quently oblong and very irregular. Such cavities are frequent in 

 the mountains of the Jura. These are generally oblong hollows, 

 like clefts, which sometimes extend to a great distance, forming 

 oblong mounds parallel to each other, with salient summits. 

 These depressions or cavities (fig. 73) have received the name of 



116 



