WILLIAM G. STEVENSON. 199 
like solar and lunar attractions and barometric fluctua- 
tions—are much less important agents than those which 
belong more directly to the structure and conforma- 
tion of the earth itself. 
We shall not, therefore, discuss the influences which 
may be exerted by rain, winds, and changes in atmos- 
pheric temperature in the causation of seismic phenom- 
ena, because their influence is not prepon derating. But 
reference will be made to some of those influences which 
are known to possess more direct seismic potency. 
Mr. Poulette Scrope ascribed most earthquakes to 
‘‘the snap and jar occasioned by the sudden and violent 
rupture of solid rock-masses, and, perhaps, the instan- 
taneous injection into them of intumescent molten 
matter from beneath.” 
Mr. Mallet, however, argues that, on mechanical prin- 
ciples, such rock fractures “¢gould produce only very 
weak impulses;”’ but, nevertheless, thinks that some 
earthquakes ‘‘may be due to the movement and 
erushing of rock-masses by tangential pressures, pro- 
duced by secular cooling of the earth.” 
Whatever may be the agencies or causes which pro- 
duce land elevations and depressions by ,which, from 
undue strain or tension, cracks or faults are caused in 
the bed-rocks of the earth, may be regarded as seismic 
causes also; for faulting cannot take place without 
causing shocks which can be felt, and whose severity 
will be measured by the magnitude of the rock-fracture 
and dislocation. 
In minor earthquakes, the fractures do not extend to 
the surface ; and so a flexure, rather than a marked dis- 
placement, only may be observed. When the fracture 
and displacement are beneath the ocean, of course we 
can only feel the shock, or observe the water-wave ; but 
cannot discern topographical changes, except in the 
lifting up of islands or coast lines, or in their depression. 
8s 
