OF VOLCANIC ROCKS. 71 
which assume either all or a part of the massive crystalline rocks composed of silicates 
to have originated by the metamorphism of sediments) we will first point ont some 
reasons against its general tenor, which may be conclusive in regard to those rocks 
the eruptive nature of which is almost generally conceded, while we will have to 
bring some additional arguments against its special application to those which are often 
designated as ‘‘ hypogene ” or ‘ plutonic.” 
The metamorphic theory starts from the assumption of an alternating progres- 
sion from the center of the earth, and recession towards it, of the chthonisothermal 
planes, the former being caused by sedimentary deposition, the latter by denudation. 
It is then argued that, by the progression of these planes, sedimentary strata would 
acquire a more and more elevated temperature, and, being permeated by water, would 
be metamorphosed, and finally rendered liquid. It is demonstrated that heat, generated 
by the plication of the strata in the lowest and central part of an area of subsidence, 
would aid in promoting these changes, which would end in the rupturing of the crust 
and the protrusion of liquid matter. Eruptive activity should, according to these 
views, be confined to areas of subsidence, in particular to their central portions. Geo- 
logical observation does not favor this conclusion, since the emission, at least of the 
voleanic rocks, has taken place on the borders of those areas, on high table lands, and, 
in general, in places which have undergone elevation before and since the time of the 
first commencement of the eruptive activity. 
It would appear that the experiments of Daubrée should not be too freely 
applied to reasoning on processes within the shell of sedimentary rocks. They have 
been made with relatively large quantities of water, such as can scarcely be expected 
to be present in solid rock at some distance below the ground. Supposing that that 
quantity naturally enclosed in it would, under great pressure, cause its fusion, then 
no reason can be adduced why there should not prevail a liquid state of all matter at a 
limited depth below the surface, and over extensive regions, if not over the whole 
globe. That such is not the case, is evident from the want of any signs of subterranean 
tides. Leaving this difficulty out of consideration, another presents itself concerning 
the periodicity of eruptive activity. If processes such as those suggested by the 
adherents of the metamorphic doctrine were its cause, then it would be impossible to 
give an explanation, why there have been long eras of rest intervening between others 
of violent eruptive activity, or why the latter was of general distribution over the globe 
during the Tertiary era, and preceded by a period of rest which was probably no less 
general. The phenomena of vulcanism might have manifested themselves at a certain 
time more in one country than in another, if metamorphism had been their cause, since 
their principal theater would be constantly shifted to the places of the most violent 
metamorphic action ; yet they should, at least in the aggregate, have been continuous. 
These objections, however, against the metamorphic theory of eruptive rocks 
are of little weight when compared with another, respecting their nature as chemical 
24 Tt is needless to enter here upon a discussion of the causes and effects attributed to these phases in the “ flow of 
heat” by the adherents of the metamorphic theory, since this topic has been made lately the subject of a paper by the skillful 
hand of Professor Dana. 
(109) 
