R. A. Daly — Secondary Origin of Certain Granites. 185 



Art. XXIII. — The Secondary Origin of Certain Granites; 

 by Reginald A. Daly, Ottawa, Canada. 



[Published by permission of the Chief Commissioner for Canada, Interna- 

 tional Boundary Surveys.] 



Contents. 

 General thesis of the paper. 



A. The Sills of the British Columbia (International) Boundary. 



The -Movie Sill. 

 Field Hypothesis. 



B. Occurrences in Minnesota. 



(«) Pigeon Point. 



(6) Governor's Island. 



(c) Lake Superior islands and Logan sills. 



(d) Cook County, Lake County and other localities. 



C. The Sudbury intrusive sheet. 

 Synthetic discussion. 



Magmatic assimilation. 



Summary, 

 Asymmetry of the intrusive bodies. 

 Magmatic Differentiation. 

 General Application. 



General thesis of the paper. — Igneous rocks originate in 

 magmas. The discovery of the laws governing the immediate 

 derivation of such rocks from their parent magmas is, there- 

 fore, not the final aim of the geologist. He is logically com- 

 pelled to refer rocks themselves to the yet more fundamental 

 problem of the origin of igneous magmas. Whence come the 

 raw materials of basalt, gabbro, porphyry or granite 2 



One of the earliest answers to this question has been grad- 

 ually assuming a systematic statement in the form of the 

 " assimilation theory". This theory holds that some igneous 

 rocks are derived from the compound magmas formed by the 

 local fusion of solid rock in molten rock of a different chemical 

 composition. The process can be imitated in the laboratory 

 furnace, and has certainly operated on many igneous contacts 

 in nature. Yet one of the very latest utterances of one of the 

 world's greatest petrologists reads thus : " The untenability 

 of the 'assimilation' or fusion theory I regard as definitely 

 proved."* On the other hand, a no less well known authority 

 claims assimilation on a large scale as a necessary stage in the 

 preparation of the Christiania granite. f Brogger and many 

 of his followers hold that the contact phenomena of this granite 

 show that the assimilation theory breaks down even when 

 applied to a most favorable case. 



*"Die Unhaltbarkeit der 'Assimilations'- oder Einschmelzungs-Theorie 

 betrachte ich als endgultig bewiesen." — J. H. L. Vogt, Die Silikatschmelz- 

 losungen, Part II., Christiania, 1904, p. 225. 



I F. Loewinson-Lessing, Comptes Eendus, 7th Session, International Geo- 

 logical Congress, 1899, p. 369. 



Am. Jour. Sci. — Fourth Series. Vol. XX, No. 117. — September, 1905. 

 L3 



