EARTHQUAKES. 175 



tiers, thrown up to a height of 50 — GO feet, fell back partly 

 into the orifice of eruption, while a part of them covered the 

 walls of the eonc. The regularity of such a phenomenon 

 renders its observation free from danger. The constantly- 

 repeated small earthquake was quite imperceptible beyond 

 the crater — even in the Atrio del Cavallo and in the Her- 

 mitage Del Salvatore. The periodicity of the concussion 

 shows that it was dependent upon a determinate degree of 

 tension which the vapors must attain, to enable them to 

 break through the fused mass in the interior of the cone of 

 cinders. In the case just described no concussions were felt 

 on the declivity of the ashy cone of Vesuvius, and in an ex- 

 actly analogous but far grander phenomenon, on the ash- 

 cone of the volcano of Sangai, which rises to a height of 

 17,00G feet to the southeast of the city of Quito, no trem- 

 bling of the earth* was felt by a veiy distinguished observer, 

 M. Wisse, when (in December, 1849) he approached within a 

 thousand feet of the summit and crater, although no less than 

 2G7 explosions (eruptions of cinders) were counted in an hour. 



A second, and infinitely more important kind of earth- 

 quake, is the very frequent one which usually accompanies 

 or precedes great eruptions of volcanoes — whether the vol- 

 canoes, like ours in Europe, pour forth streams of lava ; or, 

 like Cotopaxi, Pichincha, and Tunguragua of the Andes, 

 only throw out calcined masses, ashes and vapors. For 

 earthquakes of this kind the volcanoes are especially to be 

 regarded as safety-valves, as indicated even by Strabo's ex- 

 pression concerning the fissure pouring out lava near Le- 

 lante, in Eubcea. The earthquakes cease when the great 

 eruption has taken place. 



Most widely! distributed, however, are the ravages of the 



* The explosions of the Sangai, or Volcan de Macas, took place on 

 an average every 13"'4; see Wisse, Comptes rendus de VAcad. des 

 Sciences, tome xxxvi., 1853, p. 720. As an example of commotions 

 confined within the narrowest limits, I might also have cited the re- 

 port of Count Larderel upon the lagoons in Tuscany. The vapors 

 containing boron or boracic acid give notice of their existence, and of 

 their approaching eruption at fissures, by shaking the surrounding- 

 rocks (Larderel, Sur les etablissements industriels de la production 

 d'acide boracique en Toscane, 1852, p. 15). 



f I am glad that I am able to cite an important authority in con- 

 firmation of the views that I have endeavored to develop in the text. 

 " In the Andes the oscillation of the soil, due to a volcanic eruption, 

 is, so to speak, local, while an earthquake, which, at all events in ap- 

 pearance, is not connected with any volcanic eruption, is propagated 

 to incredible distances. In this case it has been remarked that tho 



