r9o6.] SEE— THE CAUSE OF EARTHQUAKES. 325 



whether of continued or isolated character, with their numerous 

 jutting spurs and cross ranges, and isolated peaks, which are well- 

 nigh unintelligible on any other hypothesis. And lastly it shows 

 that all mountains are alike inside, whether they burst open and 

 become volcanoes or remain intact. 



A theory presenting so many desirable points should have a 

 strong claim to acceptance. 



In this connection one geological term in extensive use might 

 perhaps be explained. We refer to the phrase Line of Weakness of 

 the earth's crust, which was employed by Leopold von Buch to 

 explain the arrangement of volcanoes along the seashore. It forms 

 wherever the sea stands some time, especially if the sea is deep, 

 because the explosive paroxysms of steam work under the edge of 

 the sea, but not under the land, and, therefore, " lateral thrusts " 

 from the sea begin, while they cease on the land; the result is an 

 injection of the coast line from the direction of the sea, and moun- 

 tains and volcanoes are upraised, according to the intensity and 

 especially the difference of these forces, from the sea and land, and 

 their duration. 



It is not without significance that the height of the mountains 

 are in general proportional to the depth of the adjacent sea, because 

 the forces of injection depend upon the depth, and the elevations 

 produced are proportional to the intensity of these forces. When 

 the sea recedes, however, the extent of the land gained is propor- 

 tional to the shallowness of the water, and hence arise the large flat 

 plains in many countries. This explains the arrangement of moun- 

 tains and volcanoes along the sea coast, which has, therefore, been 

 called a line of weakness in the earth's crust. As a matter of fact, 

 any line will prove to be weak where the sea stands for a long time, 

 for mountains and volcanoes will be upheaved there. Thus I con- 

 ceive that there is originally no such thing as a line of weakness in 

 the crust, and we may with advantage dispense with that unfortu- 

 nate term. This seems the more advisable, since the earth behaves 

 as a solid, and local weakness developed in the formation of moun- 

 tains has little effect at a distance, except in volcanic regions or 

 ocean troughs, which act together sometimes throughout their whole 

 extent. 



