“WILLIAM G. STEVENSON. 195 
Fortunately such sea-waves are not the common at- 
tendants of earthquakes, for, out of fifteen thousand 
earthquakes observed along various coast lines, there 
have been but one hundred twenty-four sea-waves pro- 
duced thereby. 
Although the shock is felt so distinctly on the earth’s 
surface, it is the testimony of many observers that, 
deep underground, itis either not felt at all, or, if felt, 
it is generally very feeble. Such has been the evidence 
of those who have been in deep mines at the times when 
severe earthquakes have occurred. 
The explanation of this is thought to be either be- 
cause of a ‘‘smaller amplitude of motion in the solid 
rocks beneath the surface as compared with the extent 
of motion on the surface, or else the disturbance is, at a 
distance from its origin, practically confined to the 
surface.”’ 
The depth at which earthquakes originate varies— 
it is believed—from a mile and a half, as in the earth- 
quake of Yokohama in 1880, to fifty miles asin Owen’s 
valley earthquake in 1872. Mallet’s calculations, how- 
ever, give the limiting depth at thirty miles, but his 
calculations were based upon the idea, first propounded 
by Mitchell in 1700, that ‘‘the impulsive effect of an 
earthquake has an intimate relationship with the height 
of neighboring volcanoes, the column of lava supported 
on a volcanic cone being a measure of the internal press- 
ure tending to rupture the adjacent crust of the earth.” 
Prof. Milne, however, shows that this argument is 
subject to such qualifications, that it cannot have a gen- 
eral application. He says that the connection between 
particular earthquakes and volcanoes is not always 
specially apparent; and, while the column of liquid 
lava in the cone of a voleano may ‘‘ measure the pres- 
sure upon the crust of the earth in the immediate vicinity 
of the cone,” it may not be a correct index of the pres- 
79 
