79 
Dutton, | e [April 7, 1871. 
below the surface, became softened or lightened by the combined agencies 
described, so as to be specifically lighter than the average mass of over- 
laying rock. Ifa vent or fissure could be found, such a plastic mass 
would inevitably follow the laws of the equilibrium of fluids, and would 
not only rise up into the chasm, but overflow. Putting the problem into 
another form, the heavier over-lying mass would sink into the lighter 
semi-fluid beneath, and drive it upwards. It is a well known fact, that 
the lavas are all of small specific gravity. Indeed, were it otherwise, 
Lieut. Dutton thought that the overflow of a lofty voleano like AStna or 
Mauna Loa, would be impossible; for a column of dense material of such 
a height, exerting its pressure upon its subterranean reservoir, would 
raise the overlying strata, instead of rising above them. But, in truth, 
the superior strata are doubtless heavier, and exert a greater pressure 
upon the reservoir than the lava itself. 
Ina similar manner Lieut. Dutton sought to explain the intrusion of 
traps, trachytes and basalts. These rocks were probably lighter than 
those which originally overlaid them, and forced their way through weak 
places to the surface. The traps, basalts and porphyries,—at least such 
porphyries as may be called intrusive—though they are unquestionably 
altered sediments, are for the most part amorphous, and not crystalline. 
They were evidently altered at a comparatively low temperature, and at 
no very great depth. They do not appear to affect the strata into which 
they are intruded, and withal, are less highly metamorphic than gneiss or 
marble. Water seems to have been the chief agent in their transforma- 
tion, and they may have been forced upward in asoft condition, and upon 
being relieved of the pressure, parted with the greater portion of this 
water, The traps and basalts also exhibit many planes of cleavage, 
with very perceptible interstices, and these interstices would seem to be 
much wider than could be accounted for by the contraction of cooling. 
He stated that he had often noted this fact, and was decidedly of the 
opinion that the contraction of these rocks by loss of heat, could by no 
means account for the entire width of such plans of cleavage, and be- 
lieved that it was in great part due to the loss of water, which had once 
rendered them plastic. 
If these views be correct, then we ought to expect that voleanic regions 
will be confined to those areas which have recently been regions of marked 
elevation. And we find this to be the case. In America, the whole extent 
of the Rocky Mountains and of the Andes, so far as known, was covered 
by the ocean at the beginning of the Tertiary period. The elevation of 
the Rocky Mountains was probably earlier than that of the Andes, and 
sooner completed. Hence, while the former was the scene of an unparal- 
leled amount of volcanic action during the Pliocene and Miocene, and is 
now nearly, or quite, quiescent, except in Southern Mexico, the Andes 
still abound in active volcanoes. The East Indian voleanice regions are 
all of Tertiary formation, as are those of the Mediterranean and the 
Auvergne. 
