EARTHQUAKE AREAS OF THE EARTH 447 



The greatest of the Italian earthquakes have been the result of earth 

 fracture accompanied by a succession of destructive shocks. They 

 have been due to the fact that Italy lies within an area characterized 

 by a state of unstable equilibrium. Its mountains and the most lofty 

 mountains of the world, the Himalayas, were uplifted during the last 

 great period of mountain-building — for it must be remembered that 

 not all mountains are of the same age — and that the rocks of the earth 

 had been folded into great arches, upon which were superinduced other 

 folds, long before the uplift of the Himalayas. In other words, most 

 of the highest mountains of the earth are much younger geologically 

 than the older and less lofty ranges, just as the Eockies are vastly 

 younger than the mountains of the Appalachian region, while they, in 

 turn, are much younger than the Blue Eidge — itself younger than the 

 remnants of other mountains which, however, in the attribute of eleva- 

 tion, are mountains no longer — since now only their upturned eroded 

 roots remain marking sites where their masses formerly stood in the 

 long distant past, as great, towering topographic features. Old moun- 

 tains thus become bevelled off and their sediments may again, in the 

 evolution of the continents, be uplifted elsewhere after eons of ages 

 to form new and even much higher mountains than before. 



The cause of the movements of the earth which produce mountains 

 is a long story in itself — and so its discussion here must needs be 

 omitted. For our present purpose it will suffice to say that such move- 

 ments do occur. Furthermore, they proceed very slowly, so slowly that 

 while such deformations are in progress they may be at such a rate as 

 to be wholly imperceptible and yet, acting through a vast period of time, 

 be sufficient to lift rock formations from beneath the sea to thousands 

 of feet above sea level. In the course of the uplift, accompanied as it 

 is by folding, obviously enormous stresses are developed and from time 

 to time rocks give away and Assuring and faulting ensue ; and when this 

 happens the shocks that are felt are said to be the result of what is 

 termed a tectonic earthquake. Later on other movements may occur 

 along a line of weakness produced in this way, and at different points 

 along this line and at different times, just as the California earthquake 

 was the next to the last of a series of eight movements along a well- 

 defined line of crustal fracture. The movements which culminated in 

 the California earthquake began in the north off the coast of British 

 Columbia. The first and third of the series occurred in this region in 

 September, 1899, and October, 1900. They were then felt off Central 

 America in January, 1900, and April and September, 1902. After a 

 lapse of over three years, sufficient stresses had accumulated to inaugu- 

 rate another movement, upon the southern continuation of the same 

 line. This shock was noted off southern South America on January 

 31, 1906; and then several months later came the California earthquake 



