EARTHQUAKES. 211 



column of smoke, which had been observed to rise for months 

 together from the volcano of Pasto, in South America, sud- 

 denly disappeared, when, on the 4th of February, 1797, the 

 province of Quito, situated at a distance of 192 miles to the 

 south, suffered from the great earthquake of Riobamba. 

 After the earth had continued to tremble for some time 

 throughout the whole of Syria, in the Cyclades and in Eubea, 

 the shocks suddenly ceased on the eruption of a stream of hot 

 mud on the Lelantine plains near Chalcis.* The intelligent 

 geographer of Amasea, to whom we are indebted for the 

 notice of this circumstance, further remarks : " since the 

 craters of Etna have been opened, which yield a passage to 

 the escape of fire, and since burning masses and water have 

 been ejected, the country near the sea-shore has not been so 

 much shaken as at the time previous to the separation of 

 Sicily from Lower Italy, when all communications with the 

 external surface were closed." 



We thus recognise in earthquakes the existence of a vol- 

 canic force, which although everywhere manifested, and as 

 generally diffused as the internal heat of our planet, attains 

 but rarely, and then only at separate points, sufficient intensity 

 to exhibit the phenomenon of eruptions. The formation of 

 veins, that is to say, the filling up of fissures with crystalline 

 masses bursting forth from the interior (as basalt, melaphyre, 

 and greenstone), gradually disturbs the free intercommuni- 

 cation of elastic vapours. This tension acts in three different 

 \vays, either in causing disruptions, or sudden and retroversed 

 elevations, or, finally, as was first observed in a great part of 

 Sweden, in producing changes in the relative level of the sea 

 and land, which, although continuous, are only appreciable at 

 intervals of long period. 



Before we leave the important phenomena which we have 

 considered, not so much in their individual characteristics as 

 in their general physical and geognostical relations, I would 

 advert to the deep and peculiar impression left on the mind by 

 the first earthquake which we experience even where it is not 



* Strabo, lib. i. p. 100, Casaub. That the expression TTT/XOU Sicnrvpov 

 -iroTctfiov does not mean erupted mud, but lava, is obvious from a passage 

 in Strabo, lib. vi. p. 412. Compare Walter, in his Abnahme der vulka- 

 ni$chen Thdtigkeit in historischen Zeiten (On the Decrease of Volcanic 

 Activity during Historical Times), 1844, s. 25. 



p 2 



