Earthquakes. 161 



of the United States, in 1843, ran parallel to the volcanic 

 chain in Mexico. In South America they roll along the 

 Andes. That of 1797 left its tracks along a westerly line 

 from I'unguragua through Pelileo and Giiano. It is a lit- 

 tle singular, that while the late trembling at Quito seemed 

 to come from the north, the great shock in Peru preceded 

 that ni Ecuador by three days. Though the origin of 

 earthquakes is deep-seated, the oscillation is mostly super- 

 ficial, as deep mines are little disturbed. The most damage 

 is done where the sedimentary^ plains abut against the hard, 

 upturned strata of the mountains. The shock is usually 

 brief. That of Caracas lasted fifty seconds, that of Lisbon 

 six minutes ; but Humboldt witnessed one in South Amer- 

 ica which continued a quarter of an hour. 



Several hypotheses have been advanced to account for 

 earthquakes. Rogers ascribes them to billowy pulsations 

 in the molten matter upon which the flexible crust of the 

 earth floats. Mallet thinks they may be viewed as an un- 

 completed effort to establish a volcano. Dana holds that 

 they are occasioned by the folding up of the rocks in the 

 slow process of cooling and consequent contraction of the 

 earth's crust. In this process there would occur enormous 

 fractures to relieve the tension ; tilted strata would slip, and 

 caverns give way. All this no doubt takes place ; but the 

 sudden, paroxysmal heavings incline us to refer the cause 

 to the same eruptive impulse which makes Vesuvius and 

 Cotopaxi discharge pent-up subterranean vapor and gas. 

 The most destructive earthquakes occur when the overlying 

 rocks do not break and give vent to the imprisoned gas. 

 There is some connection between volcanoes and earth- 

 quakes ; the former are, to a certain extent, "safety-valves." 

 The column of smoke from the volcano of Pasto suddenly 

 disappeared just before the great earthquake at Riobamba. 

 In the spring of 1868 Piehincha and Cotopaxi showed signs 



