D. Forbes — On Volcanos. 325 



furnish the sulphuretted hydrogen, sulphurous acid, and nitrogen 

 gases, given off; whilst the common salt in the sea-water, by its 

 action on the hot silicates in presence of steam, would eliminate 

 hydrochloric acid, and account for the appearance of it, as well as of 

 the volatile chlorides found in volcanic fumes. If we accept this 

 explanation, the chemical reactions would be but the effects and not 

 the cause of volcanic phenomena. 



The destructive effects attendant on volcanic convulsions are of two 

 different characters, viz. : those arising from the earthquakes which 

 accompany, and as a rule precede, volcanic outbreaks, and those 

 caused by the products ejected from the volcano itself. The connec- 

 tion of earthquakes with volcanos has been noted from the oldest 

 times. The earthquakes which commenced a.d. 63 were but the 

 efforts made by Vesuvius to relieve itself, which culminated in the 

 great eruption in 79 ; the same was the case in Mexico, with Jorullo, 

 in 1759 ; and with the great earthquake of 1834, in Chili, which 

 ended in the outbreaks of Osorno and three other volcanos in the Andes ; 

 and lastly, in 1868, the terrible earthquake which visited the coast of 

 Peru, and totally destroyed the cities of Arica and Icquique, was fol- 

 lowed by the eruption of Isluga, which, according to the latest news, 

 still continues. 



There seems little reason to doubt that all earthquakes are of purely 

 volcanic origin, and that volcanos themselves may be regarded as 

 so many safety-valves for blowing off the surplus steam, gases and 

 molten products from our great internal boiler; for as a rule it has 

 been observed that earthquakes either cease altogether or diminish 

 greatly in violence as soon as a neighbouring volcano has cleared 

 its throat. 



Although I have resided several years in what are called earth- 

 quake countries, and have experienced numerous and severe shocks, 

 amongst others those which resulted in the total destruction of the 

 cities of Copiapo and Mendoza, on which latter occasion some 20,000 

 inhabitants perished in the ruins, it seems to me quite impossible to 

 convey in words anything like a true picture of such a dreadful 

 catastrophe. The feeble shocks occasionally felt in England cannot 

 give even the remotest idea of what a severe earthquake is in reality ; 

 for not only are cities destroyed and whole villages swallowed up 

 in an instant, as in the case of Argure during the eruption of Mount 

 Ararat in 1840, but when situated on the coast, even when they have 

 withstood the shock itself, they may be entirely swept away by the 

 great sea wave which follows close upon it, as happened with the 

 cities of Arica and Icquique, in Peru, little more than a year ago. 



Equally terrible is the destruction caused by the showers of ashes 

 and torrents of molten rock, as in the well-known instances of Pom- 

 peii, Herculaneum, and others too numerous to mention ; but as time 

 presses, I will, in conclusion, only add some remarks upon the in- 

 fluences which such volcanic or cataclysmic forces exert upon the 

 external configuration of our globe and the features of its scenery. 

 In a lecture published some years ago, when referring to the prime 

 causes which determined the external configuration of the land in the 



