4 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. 



in a horizontal or in a vertical direction according to the tectonic conditions, and these may 

 thus affect the type of rocks produced as well as the form of the complex. 



Moreover, since both discussions show how small a proportion of the initial magmas can 

 become concentratedly alkaline, they accord with Daly's demonstration ('14) of the exceed- 

 ingly small bulk of the alkaline rocks compared with the alkali-calcic types, and lead us to 

 infer that the individual rocks of a subalkaline complex, from which concentratedly alkaline 

 rocks have been removed by differentiation, will not be noticeably poorer in alkalies than 

 those which have suffered but little differentiation. The practical uniformity of the charac- 

 ters of, e. g., felspar-basalt from alkaline and calcic complexes, (Flett's ('12) instance of a 

 "diphylitic rock type") may perhaps be explained in this manner. 1 



i Holtedahl's discussion ('19) of " Sal and Sima" rocks has not been accessible to the writer. 



