606 PROF. J. P. IDDINGS ON EXTRUSIVE [Nov. 1 896, 



35. Extrusive and Intrusive Igneous Rocks as Products of 

 Magmatic Differentiation. By Prof. J. P. Iddings, For.Corr.G.S. 

 (Bead June 24th, 1896.) 



[Plate XXIX.— Map.] 



Prof. W. C. Brogger, in dedicating to me his recent work on * Die 

 Eruption sfolge der triadischen Eruptivgesteine bei Predazzo in 

 Siidtyrol,' * has not only honoured me by this token of his friend- 

 ship, which I prize highly, but also by associating my name with a 

 work which commands my admiration, and with which I am in the 

 most hearty accord. His treatment of the igneous rocks in the region 

 of Predazzo as a generic series intermediate in composition between 

 a granite-diorite-gabbro series and a granite-syenite (elaeolite-syenite) 

 series is fully justified by their chemical and mineral composition 

 and by their geological association ; and for the order in which 

 these rocks have been erupted he has given ample evidence. The 

 intrusive character of the granular rocks is clearly established, and 

 the fantastic theory of Beyer abundantly disproved. So also the 

 discussion of Kjerulf and Michel-Levy's hypotheses, regarding the 

 melting and assimilation of overlying rocks by molten granitic 

 magmas, leaves nothing to be said in further refutation of these 

 suggestions, so far as they relate to the intrusion of igneous magmas 

 that come within the range of our investigation. To suppose that 

 such a process may have been active in the earliest period of the 

 cooling globe is quite reasonable. It implies a very highly-heated 

 magma, and shifting temperature or convection-currents. As 

 applied by Suess 2 to account for the great craters of the moon, it 

 appears highly probable. But there is no evidence that such a 

 melting has taken place in the case of the intrusion of igneous rocks, 

 except to a very limited extent. The remarks of Brogger on this 

 subject also express the conclusions of those who have studied 

 the phenomena of intrusive rocks in the United States. 



In his closing argument relating to the order of eruption, and to 

 the differentiation of igneous magmas as exemplified in groups of 

 intrusive and extrusive rocks, he reaches, however, conclusions which 

 are different from those to which a study of the igneous rocks in 

 certain parts of Western America must necessarily lead one. And 

 since he has cited the occurrence of igneous rocks at Electric Peak 

 and Sepulchre Mountain in the Yellowstone Park 3 in support of his 

 position, it is only right that I should explain, more fully than I 



1 ' Die Eruptivgesteine des Kristianiagebietes.' II. Videnskabs-selskabets 

 Skrifter I. Mathematisk-nanirv Klasse, 1895, no. 7. 



3 Suess, E., 'Einige Benerkunpen liber den Mond,' Sitzungsb. d. k. Akad. d. 

 Wissensch. Wien, vol. civ., Feb. 1895. 



3 Iddings, J. P., ' The Eruptive Rocks of Electric Peak and Sepulcbre 

 Mountain, Yellowstone National Park,' 12th Annual Eeport U.S. Geol. 

 Surv., pp. 569-664, Washington, 1891. 



