204 cooMos. 



In order to give a general delineation of the causal con- 

 nection of geognostical phenomena, we will begin with those 

 whose chief characteristic is dynamic, consisting in motion 

 and in change in space. Earthquakes manifest themselves 

 by quick and successive vertical, or horizontal, or rotatory vi- 

 brations.* In the very considerable number of earthquakes 

 which I have experienced in both hemispheres, alike on land 

 and at sea, the two first-named kinds of motion have often ap- 

 peared to me to occur simultaneously. The mine-like explo- 

 sion — the vertical action from beiow upward — was most strik- 

 ingly manifested in the overthrow of the toAvn of Riobamba 

 in 1797, when the bodies of many of the inhabitants were 

 found to have been hurled to Ciillca, a hill several hundred 

 feet in height, and on the opposite side of the River Lican. 

 The propagation is most generally effected by undulations in 

 a linear direction,! with a velocity of from twenty to twenty- 

 eight miles in a minute, but partly in circles of commotion oi 

 large ellipses, in which the vibrations are propagated with 

 decreasing intensity from a center toward the circumference. 

 There are districts exposed to the action of two intersecting 

 circles of commotion. In Northern Asia, where the Father 

 of History, $ and subsequently Theophylactus Simocatta,^ de- 

 scribed the districts of Scythia as free from earthquakes, I 

 have observed the metalliferous portion of the Altai Mount- 

 ains under the influence of a two-fold focus of commotion, the 

 Lake of Baikal, and the volcano of the Celestial Mountain 

 (Thianschan).ll When the circles of commotion intersect one 

 another — when, for instance, an elevated plain lies between 

 two volcanoes simultaneously in a state of eruption, several 

 wave-systems may exist together, as in fluids, and not mu- 

 tually disturb one another. We may even suppose interfer- 



* [See Daubeney On Volcanoes, 2d ed., 1848, p. 509.]— Tr. 



t [Ou the linear direction of earthquakes, see Daubeney On Volcu' 

 noes, p. 515.] — Tr. 



X Herod, iv., 28. The prostration of the colossal statue of Memnou, 

 which has been again restored (Letronue, La Statue Vocale de Memnon, 

 1835, p. 25, 26), presents a fact in oppositioii to the ancient prejudice 

 that Egypt is free from earthquakes (Pliny, ii., 80); but the valley of 

 the Nile does lie external to the circle of commotion of Byzantium, tho 

 ■ Archipelago, and Syria (Ideler ad Aristot., Meteor., p. 584). 

 ' $ Saint-Martin, in the learned notes to Lebeau, Hist, du Bos Empire, 

 t. ix., p. 401. 



II Humboldt, Asie Centrale, t. ii., p. 110-118. In regard to the dif* 

 fcrence between agitation of the surface and of the strata lying beneath 

 it, see Gay.Lussac, in the Annates de Chimie et de Physique, ' xxii.. p 

 W9. 



