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 
chap ters Y 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 
XII)_. 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 
lias 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.” 
Xfif 
The subject of earthquake distribution or seismic geography is treated 
in two chapters (XIV an d XV ), and the final chapter (XVI) is devoted 
to seaquakes. 
As an observer, Dutton 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. Rept., 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 matters from his mind, even to 
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; but when 
ready he penned all his own manuscripts rapidly under the stimulus of 
