326 Arthur Holmes — Petrology of North- Western Angola. 



The nature of the groundmass and the absence of felspar, in con- 

 junction with the melanocratic habit of the rock, suggest that it should 

 be placed among the monchiquites. It differs, however, from most 

 monchiquites in having a somewhat lower proportion of the mafic 

 minerals, and more vitally in the total absence of olivine. The 

 naming of this rock raises the important question whether the 

 presence or absence of olivine in a rock should affect its name and 

 classificatory position. The division of igneous rocks on a silica 

 saturation basis, as recently developed by Professor Shand, 1 seems to 

 afford a sound basis for a more satisfactory mineralogical classi- 

 fication than has yet been proposed. In it rocks containing quartz, 

 felspathoids, or olivine, would be sharply and naturally distinguished 

 from any in which those critical minerals were not present. This 

 distinction is already frequently made. Thus we have basalt and 

 olivine basalt, tephrite and basanite, nephelinite and nepheline basalt 

 (a better name for which would be olivine-nephelinite). The natural 

 distinction based on the principle of saturation, 2 and actual precedent 

 are therefore both against any extension of the existing term monchi- 

 quite to include rocks free from olivine. The difficulty may be evaded 

 by defining monchiquite so that olivine shall not be an essential mineral, 

 leaving the term olivine monchiquite for the original monchiquite of 

 Hunter and Hosenbusch. 3 The only alternative appears to be that of 

 proposing a new name. The term Fourchitef which is already 

 available, cannot be applied to the Angola rock, since it was given 

 to a rock which contains 75 per cent of titaniferous augite and is 

 conspicuously poor in alkalies. The multiplication of new names 

 within a group like that of the monchiquites is greatly to be 

 deprecated, and since there are already some other monchiquites 

 which contain no olivine,' and yet which are certainly not fourchites, 

 it would be an innovation of considerable advantage to adopt the first 

 suggestion and speak of monchiquites and of olivine monchiquites. 

 Not only would precision be gained, but petrologists, already over- 

 burdened with varietal names, would be saved the influx of fresh 

 ones as geological exploration continues. The principle is one which 

 has been applied to many other groups of rocks, and which could be 

 still further extended. 



It is proposed, then, to classify monchiquitic rocks on a mineralogical 

 basis as follows, the term monchiquite alone implying the presence 

 of phenocrysts of pyroxene and amphibole in approximately equal 

 proportions. 



1 Geol. Mag., 1913, p. 508; 1914, p. 485. See also Scott, Geol. Mag., 

 1914, p. 319; 1915, p. 160. 



2 The American Quantitative Classification fails to observe this principle, 

 and the result is somewhat unfortunate, particularly in Order 5 (Q/F or 

 L/F < 1/7) which includes rocks that may contain felspars, or felspars with 

 quartz, or felspars with felspathoids. 



J "Ueber Monchiquit, etc. " : Tscherm. Min. Pet. Mit., xi, p. 445, 1890. 

 See also Evans, Q.J.G.S., lvii, p. 38, 1901. Eosenbusch did not consider 

 olivine to be an essential constituent (Element e der Gesteinslehre, 1898, 

 p. 233). 



4 Williams, Arkansas Geol. Surv. Ann. Eep., vol. ii, p. 107, 1890. 



