o64. . +. 2 DYNAMICAL GEOLOGY. - 
ative depth of the volcanic foci, their linear arrangement, and their 
occurrence along lines of dominant elevation become, he contends, 
‘intelligible under this hypothesis. For since the crushing in of the 
crust may occur at any depth, the volcanic sources may vary in 
depth indefinitely ; and as the crushing will take place chiefly along 
lines of weakness in the crust, it is precisely in such lines that 
crumpled mountain-ridges and volcanic funnels should appear. 
Moreover, by this explanation its author seeks to harmonize 
the ‘discordant observations regarding variations in the rate of 
increase of temperature downward within the earth, which have - 
already been cited and referred to unequal conductivity in the 
crust. (p. 47). He points out that in some parts of the crust the 
crushing must be much greater than in other parts; and since the 
heat “is directly proportionate to the local tangential pressure which 
produces the crushing and the resistance thereto,” it may vary 
indefinitely up to actual fusion. So long as the crushed rock 
remains out of reach of a sufficient access of subterranean water, there 
would, of course, be no disturbance. But if, through the weaker 
parts, water enough should descend and be absorbed by the intensely 
hot crushed mass, it would be raised to a very high temperature, and, 
on sufficient diminution of pressure, would flash into steam and 
produce the commotion of a volcanic eruption. 
This ingenious theory requires the operation of sudden and violent 
movements, or at least that the heat generated by the crushing 
should be more than can be immediately conducted away through 
the crust. Were the crushing slow and equable, the heat developed 
by it might be so tranquilly dissipated that the temperature of the 
crust might not be sensibly affected in the process, or not to such an 
extent as to cause any appreciable molecular rearrangement of the 
particles of the rock. But an amount of internal crushing insuf- 
ficient to generate volcanic action may have been accompanied by 
such an elevation of temperature as to induce important changes in 
the structure of rocks. . 
There is, indeed, strong evidence that, among the consequences 
arising from the secular contraction of the globe, masses of sedimen- 
tary strata, many thousands of feet in thickness, have been crumpled — 
and crushed, and that the crumpling has often been accompanied by 
such an amount of heat and evolution of chemical activity as to 
produce an interchange and rearrangement of the elements of the - 
rocks,—this change sometimes advancing to the point of actual 
fusion. (See postea p. 808, and Book LY. Part VIII.) There is reason 
to believe that some at least of these periods of intense terrestrial 
disturbance have been followed. by periods of prolonged volcanic action 
in the disturbed areas. Mr, Mallet’s theory is thus, to some extent, — 

cludes that a cubic mile of the crust taken at the mean density would, if crushed into 
powder, give out heat enough to melt nearly 3} cubic miles of similar rock, assuming 
the melting pvint to be 2000° Fahr, 

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