206 R. A. Daly — Secondary Origin of Certain Granites. 



as applying to these localities even by Van Hise, whose rare 

 knowledge of Lake Superior geology must give his opinion 

 exceptional weight.* Even the latest text-books of geology 

 give most inadequate treatment of the doctrine though it refers 

 to one of the most important problems in the whole field of 

 geology. Doubtless the majority of penologists are to-day 

 unfavorable to the assimilation theory of granite and its rela- 

 tives except as it applies to a very limited, in point of volume 

 insignificant, modification of certain magmas at their contacts. 



Van Hise's chief argument against the contact origin of the 

 Pigeon Point granite emphasizes the fact that that rock has 

 not the chemical composition either of the sedimentary forma- 

 tion or (as especially shown in the surplus of alkalies and the 

 deficiency of iron in the granophyre-granite) of a direct mix- 

 ture of gabbro and sediments.f The much quoted argument 

 of Erogger with reference to the Norwegian granites is based 

 on a similar fact.J Many other writers have, on a similar 

 ground, excluded contact assimilation as playing any consider- 

 able part in the formation of abyssal or hypabyssal magmas. 



In practically every case the opponents of the assimilation 

 theory have treated of the assimilation as essentially a static 

 phenomenon. Each interpretation of field facts has been 

 phrased in terms of magmatic differentiation versus magmatic 

 assimilation as explaining the eruptive rocks actually seen on 

 the contacts discussed. Nothing seems more probable, how- 

 ever, than that such rocks are often to be referred to the com- 

 pound process of assimilation accompanied and followed by 

 magmatic differentiation. The chemical composition of an 

 intrusive rock at a contact of magmatic assimilation is thus not 

 simply the direct product of digestion. It is the net result of 

 rearrangements brought about in the compound magma of 

 assimilation. In the magma, intrusion currents, convection 

 currents and the currents set up by the sinking or rising of 

 xenoliths must take a part in destroying any simple relation 

 between the chemical constitutions of the intrusive and invaded 

 formations. Still more effective may be the laws of differen- 

 tiation in a magma made heterogeneous by the absorption of 

 foreign material which is itself generally heterogeneous. The 

 formation of eutectic compounds or mixtures, the development 

 of density stratification, and other causes for the chemical and 

 physical resorting of materials in the new magma ought cer- 

 tainly to be regarded as of powerful effect in the same sense. 



A second fundamental principle has as a rule been disre- 

 garded in the discussions on magmatic assimilation. The form 



* Monograph XLVII, U. S. Geol. Surv., 1904, pp. 730-733. 



fOp. cit.,p. 733. 



X Die Eruptivgesteine des Kristianiagebietes, Pt. II, 1895, p. 130. 



