494 
Van Hise—Earth Movements. 
they may be seen to be arranged in definite systems occupying 
a portion of one or more of the systems of joints of the districts. 
This is nowhere more beautifully illustrated than by the numer¬ 
ous granite dikes which have been intruded along the joints in 
the Sierra Nevada granite. In the magnificent exposures of the 
Yosemite Valley this may be beautifully seen. The sills and 
laccolites have taken advantage of the partings along bedding 
planes. 
Cause of Liquefaction. — If Mallet’s 1 theory be accepted, that 
the heat liquefying the rock for vulcanism is produced by 
the mechanical crushing of orogenic movements, the for¬ 
mation of magma is certainly due to gravity, for orogenic move¬ 
ments are the direct result of gravity, or the indirect result 
arising from tangential thrust, which in most cases is caused, 
as already explained, by the general settling of the outer part 
of the earth. However, even if this theory were accepted a& 
adequate to explain the source of magma for local volcanic action 
such as now exists, few would regard it as sufficient to account 
for the vast regional extrusions and intrusions of great periods, 
of volcanic activity such as those of the pre-Cambrian (Kewee- 
nawan) of the Lake Superior region; the Silurian of G-reat 
Britain; and the Tertiary of India, New Zealand, Abyssinia, 
Great Britain, and western America. The volcanic material of 
this last period surpasses in quantity that of any previous pe¬ 
riod. But this does not show that the extrusives of the remote 
volcanic periods were less extensive; for the further back the 
eruption, the greater the proportion of the igneous rocks which 
have been transformed into sedimentary rocks by the epigene 
agencies. The predominant lavas of regional eruptions appear 
to be of an intermediate or basic character. 
If the liquid rock is not produced from the crystallized crust 
by mechanical crushing alone, it must be supposed to be resid¬ 
ual liquid material of the earth or to be produced from highly 
heated rigid or potentially liquid rock, held in the solid state by 
great pressure. If as a result of the release of pressure due to 
1 Volcanic energy; an attempt to develop its true origin and cosmical re¬ 
lations, by Kobert Mallet: Phil. Trans. Koy. Soe. London, Vol. 163, 1873, 
p. 167. 
