410 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



The details of the processes involved in the elevation of conti- 

 nents, and just how these great land areas originated, are yet to be 

 worked out, and appear to be involved in considerable obscurity, 

 most probably connected with the terrestrial origin of the moon. 

 Perhaps our knowledge is still too incomplete for a satisfactory 

 attempt at this inquiry; but it seems not improbable that the lines 

 of thought here struck out may. eventually prove fruitful of further 

 discovery. 



§ 64. Origin of the present investigation. 



It will be seen from the foregoing account that the present in- 

 vestigation grew out of the difficulty of explaining satisfactorily 

 the rotatory motion of the San Francisco earthquake. When the 

 writer very soon became convinced that the accepted theory, which 

 explained these phenomena by dislocations, subsidences and faults, 

 was not well founded, the investigation was extended little by little 

 till it covered a number of important natural phenomena not usually 

 correlated. The order in which this extension took place is perhaps 

 of no importance, but to enable anyone who might be curious to 

 -understand the original line of thought, it may not be wholly out of 

 place to mention that the next step was to assign the explosive 

 activity of volcanoes to steam; and then the traditional reasoning 

 about these vents being safety valves was intelligible, and it became 

 clear why violent earthquakes do not predominate in volcanic dis- 

 tricts and why large tectonic earthquakes are always so violent. 



While considering the mode of release of steam pressure in 

 tectonic earthquakes, where the strata are unbroken, I was led to 

 inquire into the origin of mountains. The only explanation I could 

 find was by the theory of contraction producing folding along lines 

 of weakness in the earth's crust. It seemed to me remarkable that 

 the lines of weakness should nearly always follow the coast lines 

 of continents. Such an arrangement should not occur by chance. 

 It was easy to grasp the connection of earthquakes with volcanic 

 activity when the volcano is once formed ; and the question naturally 

 arose, how did the volcanoes and mountain chains originate? I 

 recalled that volcanic islands are always being thrown up, and 

 numerous islands had been formed in this way on the bed of the 

 sea, which are mountains under water. On the other hand most 



