502 
Van Else—Earth Movements. 
material is sufficient to raise the dense lava to the place of over¬ 
flow, and nowhere does this appear to be the case. However, 
as the lava becomes porous as the result of the formation of 
steam bubbles, it becomes lighter and may rise to the top. The pro¬ 
cess once begun, the steam bubbles may form rapidly, in which 
case there is an explosive eruption. They may form rather uni¬ 
formly, and be evenly distributed through the lava, in which case 
the lava quietly overflows. After an eruption of a given volcano 
there is ordinarily a period of relative quiescence, during which 
time the slow formation of steam bubbles may continue until 
the density of the lava is sufficiently decreased so that another 
eruption may occur. 
Thus, even in the case of ordinary vulcanism the direct action 
of gravity is given a dominant place among the causes. The 
frequent inward sagging of sedimentary rocks observed about 
ancient volcanic necks is important evidence in favor of this 
view. 
I would not underestimate the power of steam in vulcanism, 
but that its expansive power is the chief force in the transfers 
of liquid material within the earth seems to me to be wholly 
unproved. The dominating force as explained is believed to be 
that of gravity. In these earth movements, as in others, the 
resultant of the gravitative movements is earth-centerward . 1 
Growth of Continents. — I shall now consider the joint effects 
of epigene transfer of material by erosion, of the compensating 
transfer of deep-seated flow toward the continents, of differen¬ 
tial subsidence, and of vulcanism, upon the growth of continents. 
1 The outline of the processes of vulcanism above given is largely inde¬ 
pendent of other authors. However a comparison with the writings of 
Hopkins, Prestwich, Geikie and Russell will show that to a greater or 
less degree these authors have expressed views similar in various respects 
to those given. 
Researches in physical geology, by Wm. Hopkins: Phil. Trans. Roy. 
Soc., 1842, Pt. I, p. 53. 
Chemical, physical, and stratigraphical geology, by Joseph Prestwich: 
Vol. 1,1886, pp. 210-211. 
Ancient volcanoes of Great Britain, by Sir A. Geikie: Vol. I, 1897, pp. 
12 and 13. 
Volcanoes, by I. C. Russell: 1897, pp. 297-326. 
