... .... GENERAL CONCLUSIONS 117 



alkaline magma is solid rock, usually carbonate. In others, the foreign 

 matter may be gas, usually carbon dioxide, which has a juvenile origin. 

 This paper has specially discussed the first group of cases. Many of the 

 leading petrologists refuse to credit the syntectic idea with having any 

 general value in the petrogenic problem, but their scepticism is scarcely 

 justified by the facts of the field. In any case it is idle to refuse assent 

 .to the proposition that magma with the known volcanic temperatures is 

 capable of assimilating limestones and dolomites. 



His own field experience has led the writer to a position much like 

 that held by Loewinson-Lessing, who states that "bisweilen geniigt ein 

 Einschmelzen einer unbedeutenden Menge irgend eines fremden Stoffes, 

 der im Magma oder in einem Theil desselben wenig loslich ist, um einen 

 Anstoss zur Spaltung in mehr oder weniger grossem Maasstabe zu 

 geben.'^^^ The present writer would extend the idea so that large-scale 

 differentiation is regarded as often induced by assimilation of foreign 

 rock-matter which, as such, is entirely miscible with the original magma. 

 The true, compound solutions so produced differentiate according to 

 reactions other than those affecting the pure, original magma. Thus, 

 for example, augite andesite is a differentiate of basalt; nephelinite is 

 one of the differentiates from a basalt-limestone syntectic. 



The hypothesis is also in line with the older views, since it offers a spe- 

 cial instance of the action of the ^^fluides mineralisateurs," which have 

 been so long emphasized by the French workers in petrogenic theory. 



A complete genetic study should discuss the many alkaline types indi- 

 vidually. This is not attempted, for the detail of their differentiation is 

 not essential to the main thesis of this paper. An inspection of the litera- 

 ture of the tabulated occurrences shows that magmatic splitting has often 

 taken place in successive stages. That gravity may be in control through- 

 out is suggested by the conditions at Square Butte and Shonkin Sag, 

 Montana. In each of these localities alkaline syenite overlies shonkinite, 

 and both types have been derived from a magma with the composition of 

 leucite basalt. The method of the differentiation is like that in the more 

 acid syntectics of the Moyie, Pigeon Point, and Sudbury sheets; and the 

 general importance of gravitation (with separation of salic and femic 

 constituents) in magmatic splitting is attested by a multitude of other 

 facts of the field.^^ In other words, the alkaline magmas seem to obey the 

 same general laws as those affecting the more voluminous subalkaline 



11 F. Loewinson-Lessing: Compte Rendu de la VIIn>e session, Congrfes geologique Inter- 

 national, Saint Petersburg, 1897 (1899), p. 380. See also p. 358. 



" See American Journal of Science, vol. 20, 1905, p. 185, and Festschrift zum sieb- 

 zlgsten Geburtstage von Harry Rosenbusch, Stuttgart, 1906, p. 203. 



