118 R. A. DALY ORIGIN OF THE ALKALINE ROCKS 



magmas. The chemical contrasts between the two groups are explained 

 by the assimilation or syntectic theory, as that theory also recognizes the 

 principal importance of magmatic differentiation. 



The writer is not able to make Jensen^s hypothesis as to the origin of 

 alkaline rocks agree with the known facts of rock distribution and with 

 chemical relationships.^^ The general association of alkaline rocks with 

 sediments is here explained in a quite different way. Jensen's conclu- 

 sion that most of the alkaline rock eruptions are of late geological dates 

 may possibly bear on the limestone-syntectic hypothesis, since it has taken 

 much of geological time to accumulate the thick masses of calcareous 

 sediments which are cut by alkaline bodies. 



Finally, the general thesis explains the localization of rock types occur- 

 ring in the alkaline "petrographic provinces." The Norwegian, Bohe- 

 mian, Italian, Monteregian, Montanian, Texan, Australian, Tasmanian, 

 and other provinces are alkaline with respect to igneous types and calca- 

 reous with respect to their sediments; and no one of these provinces 

 lacks subalkaline eruptives. Yet explanation is not proof; this must 

 await the discovery of many new facts and the judgment of physical 

 chemists cooperating with field geologists. The alkaline group of igneous 

 rocks seems to offer a specially valuable test of any petrogenic theory. 



" H. J. Jensen : Proceedings of the Linnean Society of New South Wales, vol. 33, 

 1908, p. 491. 



