THE KEVOLUTIONS OF THE CRUST OF THE EARTH. 203 



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

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

 tion as the crust where the rupture takes place is stronger. On account 

 of its slight thickness the first crust of the 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 the 

 first sedimentary strata were still i)lastic, they followed easily the con- 

 traction of the solid crust. To-day the 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 account the more recent 

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

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

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

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

 systems of Europe, and relatively mucii more important. 



Earthquakes are phenomena which most frequently accompany all 

 modification of the crust of the globe. What we know of their nature is 

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

 elastic. The 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 ifo- 

 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 attected by this earthquake 

 extended approximately 100^ in longitude and 50"^ in 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 the 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 the 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 the points at equal distance from the center of the movement 

 are similarly affected, according to the i)rinciple that the effects of a 

 blow transmitted upon different points are under identical conditions as 

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



* Prof. Pfaff, Zdtschrift der deutsch. geolog. GeseUach., 18G0, p. 4.53. 



