ON ITS EXTEIIIOIU EARTHQUAKES. 



A column of smoke which was seen for some months to rise 

 from the volcano of Pasto in South America, suddenly dis- 

 appeared, when, on the 4th of February, 1797, the province 

 of Quito, one hundred and ninety-two miles to the south- 

 ward, was visited by the great earthquake of Riobamba. 

 Tremblings of the ground, which had long been felt over the 

 whole of Syria, in the Cyclades, and in the island of 

 Euboea, suddenly ceased when a stream of lava issued 

 forth in the plains near Chalcis.( 193 ) The celebrated 

 geographer of Amasea, from whom we have received this 

 account, adds, " Since the craters of Etna have been opened 

 through which fire issues, and since glowing masses and 

 water have been ejected from them, the lands near the sea- 

 shore have not been so often shaken as in the time when, 

 previous to the separation of Sicily from Lower Italy, all 

 the issues were closed." "We thus see that the force which 

 manifests itself in earthquakes, acts also in the phsenomena 

 of volcanoes ; but though as universally diffused as the in- 

 ternal heat of the planet, and making its presence everywhere 

 known, it is only rarely, and at insulated points, that its 

 accumulated energy produces the phsenomenon of eruption. 

 The formation of veins, *. e. the filling up of fissures with 

 crystalline masses issuing from the interior (basalt, mela- 

 phyre, and greenstone), gradually impedes the free escape of 

 the elastic fluids. They then accumulate, their tension 

 increases, and their reaction against the crust of the earth 

 shews itself in three different ways, in earthquakes, in 

 sudden elevations, or in slow and continuous elevations, 

 winch alter progressively the relative levels of the land and 

 eea. The last mode of action produces effects which are 

 only sensible at intervals of long period, and was observed 

 for the first time over a considerable portion of Sweden, 



