THE REVOLUTIONS OF THE CRUST OF TnE EARTH. 293 



height of the Himalayas, the Andes, the Alps, and the Pyrenees. Only 

 their intensity has not been always the same; tbis is greater in propor- 

 tion as the ernst where the rupture takes place is stronger. On accouut 

 of its slight thickness the first crust of tbe earth offered very little re- 

 sistance to the escape of the ignited substances, which easily forced a 

 passage through the large rifts and cracks in its surface, and, as tbe 

 first sedimentary strata were still plastic, they followed easily the con- 

 traction of the solid crust. To-day tbe thickness of this envelope is 

 estimated at more than 25 miles; its resistance, therefore, is much 

 greater, and continues to increase by new eruptions and new solidifica- 

 tions of ignited and fluid substances. On this accouut the more recent 

 geological periods have been marked by fewer of the catalysms, indicated 

 by the formation of mountain chains, but tbeir intensity and violence 

 have been proportionately greater, giving rise to tbe Andes and to tbe 

 mountains of Central Asia, which are more recent than the mountain 

 systems of Europe, and relatively much more important. 



Earthquakes are phenomena which most frequently accompany all 

 modification of tbe crust of the globe. What we kuow of their nature is 

 that they are undulations of the ground, sometimes more, sometimes less 

 elastic. Tbe starting-point of these waves is in the interior of the earth, 

 often very near the surface, (at 3,400 meters, or 11,000 feet,) sometimes 

 at a very great depth, in which case a circle is shaken, including many 

 countries and entire seas. The earthquake which, on the 31st of No- 

 vember, 1755, destroyed the city of Lisbon, was felt in Labrador, and 

 from Canada to the Carolinas, in the Antilles, Morocco, the north of 

 Italy, Germany, a part of Eussia, the south of Sweden and of Norway, 

 and England felt its influence. The district affected by tbis earthquake 

 extended approximately 100° in longitude and 50° m latitude, and com- 

 prised a surface of 325 square myriameters, (12,500 square miles.) Other 

 districts affected by earthquakes have been quite equal to this one in 

 extent. Thus tbe region of earthquakes on the Mediterranean extends 

 from the Azores to beyond Lake Baikal. 



The causes of earthquakes may then vary in nature, according to the 

 depth from which they proceed, although in general their origin is the 

 same; but, whatever may be their cause, we can, through analogy of 

 their effects, make use of some mechanical laws to explain tbe phenom- 

 ena which accompany them. 



It is known that in a homogeneous and elastic medium the sum of the 

 active forces remains constant and is propagated in all directions. In 

 applying this law to earthquakes, it follows very simply that, first, the 

 point most agitated is also the nearest to the origin of the agitation, 

 and that tbe points at equal distance from the center of the movement 

 are similarly affected, according to the principle that the effects of a 

 blow transmitted upon different points are under identical conditions as 

 the sine of tbe angle at which the surface is attained;* and, second, that 

 * Prof. Pfaff, Zeifschrift der deutsoh. geolog. Gesellsch., I860, p, 453, 



