364 SCIENCE PROGRESS 



Society of South Africa on " The Problem of the Alkaline 

 Rocks " {Proc. Gcol. Soc. S. Africa, 1922, pp. xix-xxxii) is 

 noteworthy for the suggestion that alkaline rocks arise as much 

 from deficiency of alumina as of silica in magmas, and that 

 three groups of alkaline rocks may therefore be distinguished 

 nccording to whether alumina or silica, or both, are deficient 

 relatively to the alkalies. He constructively criticises the 

 views of Jensen, Daly, Smyth, and Bowen, on the origin of 

 alkaline rocks, and brings forward a hypothesis which appears 

 to synthesise the views of these investigators ; namely, that 

 alkaline rocks as a whole are developed where magma, for any 

 reason, becomes abnormally rich in volatile constituents. 

 Shand holds that syntexis of sedimentary rocks is not required 

 to explain the formation of over-saturated alkaline rocks ; 

 but that Daly's hypothesis of reaction with limestone is proved 

 for man^' occurrences of under-saturated alkaline rocks. 



Inspired by Bowen 's work on the alnoite of Cadieux, 

 Montreal (see Science Progress, April 1922, p. 548), K. H. 

 Scheumann has published an excellent paper on the petrogeny 

 of the numerous melilite-bearing types of alkaline lamprophyres, 

 such as polzenite, bergalite, etc., which have recently been 

 described from Bohemian and German localities (" Zur Genese 

 alkalisch-lamprophyrisch Ganggesteine," Centr. f. Min. Petr. 

 Pal., 1922, pp. 495-520, 521-45). Bowen 's reaction principle 

 is applied throughout, melilite being formed by the reaction of 

 an alkaline residual liquor upon p^TOxene, biotite from olivine, 

 etc. With the absorption of lime (proved by incompletely digested 

 limestone xenoliths) monticellite is formed from olivine. A 

 useful distinction is made between rocks of accumulative type, 

 due to the aggregation in any way of earlj^-formed crystals ; 

 and fusive types, in which there has been more or less 

 complete reaction between the early minerals and the alkaline 

 residual melt. 



A paper by Per Geijer on " Problems suggested by the 

 Igneous Rocks of Jotnian and Sub-Jotnian Age " {Geol. Foren. 

 Stockholm Fork., 44, 1922, 411-43) brings forward some 

 important reflections on the subject of petrographic provinces. 

 Geijer shows that, outside the trench of weakness affected by 

 Caledonian orogeny, the igneous rocks froin post-Archasan 

 times to the Cainozoic in the great Fenno-Scandian resistance 

 block have a certain community of character. They are of a 

 distinctly alkaline cast. He suggests that this fact illustrates 

 the Harker-Becke-Prior hypothesis of the connection between 

 petrographic provinces and types of earth movement. The 

 presence of alkaline rocks in a block-faulted resistant massif, 

 such as Fenno-Scandia, does not necessarily mean that an 

 alkaline magma underlies the region, but merely that the 



