160 The Andes and the Amazon. 



long at rest. Brazil, Egypt, Russia, and Greenland are 

 comparatively free from earthquakes. But had we delicate 

 instruments scattered throughout the world, upheaval and 

 subsidence would doubtless be detected in every part of 

 the so-called terra fir ma. The sea, and not the land, is the 

 true image of stability. 



" Time writes no wrinkle on thy azure brow : 

 Such as creation's dawn beheld, thou rollest now." 



Earthquakes have occurred in every period of geological 

 history, and are independent of latitude. The first well- 

 known earthquake came in the year 63, and shattered Pom- 

 peii and Ilerculaneum sixteen years before they were over- 

 whelmed by the first recorded eruption of Vesuvius. The 

 most celebrated earthquake, and perhaps the most terrible 

 manifestation of force during the human period, was in 

 1755. The shock, which seemed to originate in the bed of 

 the Atlantic, pervaded one twelfth of the earth's surface. 

 Unhappy Lisbon stood in its path. 



An earthquake is a vertical vibration, having an undu- 

 lator}^ progression. An example of the simple bomiding 

 movement occurred in 1797, when the city of Riobamba, 

 in the Quito Valley, was buried under part of a momitain 

 shaken down by the violent concussion, and men were toss- 

 ed several hundred feet. We saw one massive structure 

 which had nearly turned a somersault. The ordinary vi- 

 brations seldom exceed two feet in height. The wave- 

 movement has a rate of from twenty to thirt}^ miles a min- 

 ute, depending on the elasticity of the rock and the eleva 

 tions on the surface. "When two undulations cross each 

 other, a rotatory or twisting motion is produced. The 

 waves are generally transmitted along the lines of primary 

 mountain chains, which are doubtless seated on a fracture. 

 The Lisbon waves moved from southwest to northeast, or 

 pai-allel to th^ mountain system of the Old World ; those 



