part 1] ANIrtYERSARY ADDRESS OF THE PRESIDENT. Iv 



The Cause and Character or Earthquakes. 



The study of earthquakes having always been recognized as one 

 of the departments of Geology, no excuse is needed for devoting 

 my address, on this occasion, to a review of the present state of our 

 knowledge of the cause and character of earthquakes, using that 

 word in the restricted, and original, sense of the disturbance of 

 the ground which is sensible to human feelings, which causes 

 alarm and destruction, and is properly that seism of the ancient 

 Greeks, from which our modern term seismolo gy is derived. This 

 explanation is necessary, for, of late years, seismology has been 

 extended to the study of a phenomenon of different character, the 

 long-distance records of disturbances, only to be detected by very 

 sensitive instruments of special construction; in some cases these 

 are clearly connected with great earthquakes — as the word is here 

 used — and by inference have been presumed to be so in all cases, 

 even when there is no independent evidence of the earthquake 

 proper. The records, regarded as records of the progressive en- 

 feeblement of the larger disturbance of the true earthquake, would 

 represent the crj^ptoseism, or unfelt earthquake, and be correctly 

 described in the observatory records as earthquakes. That they 

 are correctly so described is indisputable, if the word is taken in its 

 literal interpretation as a quaking, however feeble, of the earth ; 

 but, if the implication is added that the}^ have the same origin as 

 the greater disturbance, the correctness of the description becomes 

 doubtful. In presenting to you, some dozen years ago, the results 

 of a study of the records of the Californian earthquake of 1906, I 

 pointed out that, although the immediate origin of the earthquake 

 proper might be traced to occurrences Avhich took place in the 

 outermost parts of the Earth's crust, these were [but the secondary 

 result of a deep-seated origin, or ba thy seism, which gave rise, at 

 the same time, to the disturbance which was recorded at long 

 distances by suitable instruments. Later work and research has 

 more and more confirmed both the correctness of this interpreta- 

 tion and the conclusion that the proximate cause of great and 

 destructive earthquakes is distinct from that of the long-distance 

 records, although the two origins are connected with each other as 

 effect and cause. 



In the present state of our ignorance of the nature of the 

 bathyseism, it is difficult to give a clear and precise defi.nition 



