MEMOIR OF CLARENCE EDWARD DUTTON 15 



of the earth's crust Thus we have two groups of quakes, volcanic and tec- 

 tonic. They have in many cases distinct characteristics, and these are de- 

 scribed in chapters III and IV." 



The more important instruments used in seismometry are described in 

 chapters V and VI, and chapter VII discusses the details of seismic 

 vibratory motion and explains the four kinds of waves with which the 

 inquiry deals. 



A chapter (VIII) is given to the amplitude and period of vibration, 

 and two chapters (IX and X) to the subject of intensity. The chapter 

 on the variation of intensity points out the method of computing the 

 depth of origin of an earthquake wherever observations sufficient in num- 

 ber and accuracy can be obtained. 



"The speed of propagation of seismic vibrations is then treated (XI and 

 Xlt). No specific problem in connection with earthquakes has been more dili- 

 gently investigated, and few are so difficult as this. It is only very recently 

 that definite results upon this question have been reached. The chief trouble 

 has been the great complexity of the waves generated by an earthquake, their 

 different rates of propagation, and the difficulty of separating one kind from 

 another. Nor was is known until recently that some kinds of waves are 

 propagated through the earth-mass, while others go around it. 



"Since the speed of propagation depends wholly upon the ratio of elasticity 

 to density, it becomes an index of those properties in the materials which 

 compose the earth's interior. Chapter XIII is given to the discussion of this 

 aspect of the subject." 



The subject of earthquake distribution or seismic geography is treated 

 in two chapters (XIV and XV), and the final chapter (XVI) is devoted 

 to seaquakes. 



As an observer, Button was quick to grasp the comprehensive, though 

 not overlooking details, and in the field gave most of his attention to the 

 greater problems. As he puts it (Sixth Ann. Kept., page 198) : 



"I am fond of viewing the facts observed in the field in their relation to 

 broader and more general facts, and of marshaling them into their proper 

 places." 



His method of work in preparing his reports was determined Largely 

 by his strong imagination. He made but little use of field notes except- 

 ing for figures. Shutting out all other mailers from his mind, even io 

 the neglect of personal correspondence, and without preparing a written 

 plan or preliminary draft, he read much and discussed with his col- 

 leagues. He held the subject wholly in mind until his problems were 

 solved and results fully attained before beginning to write: hut when 

 ready he penned all his own manuscripts rapidly under the stimulus o( 



