12 DE. THOMAS STEEPY HUNT ON THE 



endeavored to détermine what he conceived to be their normal composition, and, as is 

 well known, sought to show that there exists such a relation between the proportions 

 of these various bases and the silica, that it is possible to calculate the composition of 

 any given eruptive rock from the amount of this element which it contains. He thence 

 concluded that various intermediate rocks have been produced by a mingling or amal- 

 gamation, in different proportions, of these two separated magmas. For the composition of 

 these, see farther a note to § 66. I have elsewhere discussed the history of this hypothesis, 

 and have given reasons for its rejection. 



Sartorius von Waltershausen has also objected, from another point of view, to this 

 hypothesis, and, has maintained that while there is no such distinct separation of the liquid 

 interior, as was imagined by Phillips, Durocher and Bunsen, there is nevertheless a gradual 

 passage downward from a lighter acidic to a denser and more basic liquid stratiim ; beneath 

 which still heavier metallic minerals are su.pposed by him to be ai-ranged in the order of 

 their respective densities. This view has been adopted and extended by Mr. Macfarlane in 

 his paper above cited. We shall however attempt to shew in the second part of this 

 memoir that the observed relations of acidic and basic eruptive rocks admit of a widely 

 diifereut interpretation to those above given, and one more in accordance with known 

 chemical and mineralogical facts "". 



§ 25. Returning from this digression on hypothetical notions of the earth's interior, we 

 propose to consider the exoplutonic or volcanic hypothesis of the origin of the crystalline 

 stratified rocks, according to which, as concisely stated by Naumann, the material compos- 

 ing them " has been projected from the interior, through the earth's crust, in an eruptive 

 form." Inasmuch as the matter discharged in subaerial or submarine eruptions appears 

 in part as flows of molten lava, and in part as disintegrated solid materials which, like 

 other detritus, may be arranged by water, it is evident that this hypothesis connects itself 

 with that of the Huttonian school, to which, considering the mineralogical resemblances 

 between volcanic and other crystalline rocks, it would make little difierence whether the 

 sediments required for the metamorphic process came from the disintegration of older 

 crystalline strata, from a primeval granite, or from volcanic products. The volcanic hypo- 

 thesis, except so far as consolidated lava-flows are concerned, thus becomes, as we shall 

 see, a metamorphic or plutonic-detrital hypothesis. 



As an illustration of this view, we find J. D. Dana in 1843 propounding a general 

 theory of crystalline rocks, which is essentially volcanic. In this he endeavours to shew, 

 (1) that the schistose structure of gneiss and mica-schist is not a satisfactory evidence of 

 sedimentary origin, inasmuch as exotic or eruptive rocks may sometimes take on a lamina- 

 ted arrangement ; (2) that granites without any trace of schistose structure may have had 

 a sedimentary origin ; and (3) that the heat producing metamorphic changes in sedi- 

 ments did not come from below, as supposed by the Huttonians, but through the waters 

 of the ocean, heated by the same eru]3tion which brought to the surface the materials 

 of the metamorphic rocks ; which were spread over the ocean's bottom in a disintegrated 

 form. Their comminution was supposed by Dana to be effected in one of three ways ; 

 (1) they were ejected as pyroclastic material, in the form of a sand or ash-eruption, or (2) 



"" For a discussion of the views of Phillips, Durocher, Bunsen and Streng, see Hunt, Chem. and Greol. Essays, 

 pages 3-C, 66, and 284. See also Bunsen, ^Vnn. de Chim. ot de Phys. 1853, (3) xxxviii, 215-289. 



