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 out 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 " hypogcne " 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. 24 

 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 

 volcanic 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 Daubree 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 

 arc of little weight when compared with another, respecting their nature as chemical 



24 It 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. 



inr..) 



