TD mas aa 
274 ~ DYNAMICAL GEOLOGY. . . {Book TIT 
In volcanic regions the frequent earthquakes which precede — 
or accompany eruptions are doubtless traceable to explosions of — 
elastic vapours and notably of steam. As earthquakes originate — 
also in districts remote from any active volcano, and, so far as 
observation shows, at comparatively shallow depths, these cannot 
be connected with ordinary volcanic action, though it is possible that — 
by movements of molten or highly-heated matter within the crust 
and its invasion of the upper layer, to which meteoric water in con- 
siderable quantities descends, sudden and extensive generation of 
steam may occasionally take place." In minor cases where the 
tremor is slight and local, we may conceive that the collapse of the 
roof or sides of some of the numerous tunnels and caverns dissolved 
out of underground rocks by permeating water may suffice to 
produce the observed shocks. Where, however, the area convulsed 
is large, some more potent cause must be sought. One of the most 
abvious of these is the rupture of rocks within the crust under the 
intense strain produced by subsidence upon the more rapidly con- 
tracting inner hot nucleus. In mountainous districts many different 
degrees of shock from mere tremors up to important earthquakes 
have been observed, and these are not improbably due to sudden 
more or less extensive fractures of rocks which are still under great 
strain.” Hoernes, from a study of earthquake phenomena, concludes 
that though some minor earth-tremors may be due to the collapse of 
underground caverns, and others of local character to volcanic action, — 
the greatest and most important earthquakes are the immediate con- 
sequences of the formation of mountains, and he connects the lines” 
followed by earthquakes with the structural lines of mountain-axes.2 _ 
A comparison of the dates of recorded earthquakes shows that — 
they have occurred more frequently in the winter half than in the — 
summer half of the year. Out of 656 earthquakes chronicled in 
France up to the year 1845, three-fifths took place in the winter, 
and two-fifths in the summer months. In Switzerland also they have 
been observed to be about three times more numerous in winter than 
in summer.* The same fact is remarked in the history of earthquakes | 
in Britain, The general concurrence of testimony would seem to 
show that this cannot be an accidental circumstance, though it is 
not easy to explain how mere differences of atmospheric pressure can 
affect the stability of the interior of the crust. (See the remarks 
already made in regard to Stromboli, p. 210.) | 
Section III.—Secular Upheaval and Depression. 
Besides sudden movements due to earthquake-shocks, the crust 
of the earth undergoes in many places oscillations of an extremely 
’ Pfaff, Allgemeine Geologie als exacte Wissenschaft, p. 230. 
? See postea, p. 309. Suess, Lntstehung der Alpen, Vienna, 1875. 
* “ Erdbeben Studien,” Jahrb, Geol. Reichs. xxviii, (1878) p. 448. 
* Perrey, op. cit. Perrey and D’Abbadie have likewise tried to trace a connection 
between the greater frequency of earthquakes and the moon’s nearness to the earth. 

