Vulcanism. 
493 
In the above the contractional theory of mountain-making is 
accepted. However, it is a contractional theory materially 
modified from that proposed by Prevost, Dana, and Le Conte, 
in which the cause assigned was the loss of heat due to secular 
cooling. We have seen that there are other causes for superficial 
contractions, some of which may be of equal or greater importance 
than this. These are vulcanism, the increased density and de¬ 
creased oblateness of the earth due to decreasing speed of rotation, 
change in part from a liquid to a solid crystalline condition, 
change in molecular composition, and possibly also loss of oc¬ 
cluded water and gases. 
Vulcanism .— It has been held that epiorogenic, orogenic, and 
epigene movements alike are gravitative, and that their result¬ 
ant is ever earth-centerward. It remains to see that the same 
is true of vulcanism. 
It is certain that vulcanism is a phenomenon attending epio¬ 
rogenic and orogenic movements. The living volcanoes are in 
regions of known earth movements. In regions of relative 
quiescence there are no active volcanoes. In regions in which 
vulcanism has been prominent in the past, the field evidence is 
clear that these were also regions of simultaneous crustal move¬ 
ments. Moreover, it is certain that rising lava takes advantage 
of openings formed by earth movements. The rows of active 
volcanoes are presumably located along zones of faulting or 
jointing. The dikes intruded during ancient periods of vulcanism 
generally conform to the faults and joints, and in many districts 
of mountain-evolution, by 0. Davison, Geol. Mag., new ser., Vol. II, 1895, 
pp. 308-309), that Reade does not sufficiently explain how the effects of 
heat are to be concentrated along mountain ranges, and more important 
than this, the heat causing the expansion by the rise of the isogeotherms 
must be derived from somewhere else, and if derived from somewhere 
else, there must be contraction corresponding in amount to the expansion 
where the mountain ranges are formed. Further the question is perti¬ 
nent as to how far the expansion due to the rise of the isogoetherms is 
compensated for by condensation of the unconsolidated sediments also 
resulting from the same cause. Possibly the rise of the isogeotherms may 
be the real cause, among others, for the localization of mountain changes, 
but even if so, it is plainly but auxiliary to the contractional theory 
of mountain-making. 
