490 
Van Rise—Earth Movements. 
may be more basic than the average, and such undoubtedly have 
a greater specific gravity than the average of the known crust. 
Also, in many places the crust has a lower specific gravity than 
the average. At localities where either or both of these condi¬ 
tions obtain this would be unfavorable to the rise of the magma 
to the surface. Furthermore, the friction of viscous magma 
upon the walls of the orifices and within itself, during uprise, 
is great. Because of the approximate balance in density between 
magmas and the known crust, because of friction, and because 
many of the openings entered by magma do not extend to the 
surface, by far the larger part of the material which starts on 
an upward movement is probably stayed before it reaches the 
.surface. The amount of material which does reach the surface 
is indeed vast, but this amount is believed to fall far short of 
the simply enormous quantity of igneous material which stops 
within the upper crust of the earth as great batholites, laccolites, 
sills, and dikes. 
It has been said that a part of the magma does reach the sur¬ 
face. During regional eruptions it wells forth in enormous 
quantities from numerous long fissures or from throat-like ori¬ 
fices and floods the country. The amount thus poured forth at 
warious periods and in different regions has been sufficient to 
bury thousands or tens of thousands of square miles to a depth 
of thousands of feet. 
Although the absolute amount of material emitted by active 
volcanoes is very large, it is indeed small compared with 
regional extrusions. The material extruded in late Tertiary 
time is probably far greater than the amount that would 
be thrown out during an entire era at the present rate. 
During the regional eruptions great numbers of volcanic 
mountains, similar to those now in action, are also built up, 
and in a manner similar to the present volcanic mountain 
building. This is especially the case in the last epoch of a 
period of regional volcanic activity. In the Cascade region it 
was at this stage that the great volcanic mountains, such as 
Rainier, Hood, Helens, Three Sisters, Jefferson, Adams, and 
others were piled up. 
As already explained, it is believed that the dominant force 
