. W. Hilgard on Mallet’s Theory of Vulcanicity. 587 
equally the heterogeneous couches of which, on any hypothe- 
sis, the solid portion of the earth must be composed ; it will be 
difficult to persuade geologists of the actual existence of the 
“preternatural rigidity,” until every reasonable hypothesis that 
can dispense with this assumption shall have been exhausted. 
mong the objections raised by geologists, the first and 
apparently gravest was that of Forbes (Nature, Feb. 6, 1872), 
who argues the untenableness of Mallet’s theory on the ground 
of the asserted general identity of composition of volcanic 
ejecta. In fact, from Mallet’s point of view it would seem, that 
lavas might have the composition of any fusible rock whatso- 
ever, in whase strata the crushing might happen to occur; and 
hence that, if taking place within the sedimentary strata, there 
ought to be a very great diversity between the ejecta of differ- 
ent vents. 
upon the crust; and hence that it is not unreasonable to assume 
that a pressure sufficiently great to produce fusion may never 
occur within the limits of the sedimentary strata, albeit other 
manifestations of subterraneous thermal action may not be 
wanting. It is true that, on the whole, Mallet’s memoir leaves 
upon the reader's mind the impression that he seeks the source 
of voleanic action at depths sufficiently shallow to justify in 
a measure the objection raised by Forbes; although he ex- 
pressly declares that with our present data, the determination 
of the points at which the maximum of crushing effects occurs 
is impossible. ee s 
Similar considerations apply to the objection raised oy F. 
W. Hutton (Nature, Nov. 27, 1878), that “faults show no heat- 
ing effects, even where considerable crushing has taken place. 
The pressure under which the faulting occurred may have 
tion. No matter how great the dislocation or crushing, no 
great increase of demperalure can occur if it takes place siowly, 
