154 A. Holmes & H. F. Harivood — Picrite, Mozambique. 



the two rock-types are very similar. la one case (sample No. 7) 

 Mr. Goodchild remarks: "The felspars not infrequently show 

 micrographic intergrowth with the pyroxene, but on the whole are 

 interstitial," and in the photo-micrograph of the rock (Fig. 9) it can 

 be seen that the correspondence in texture goes even further, for the 

 olivines are markedly porphyritic. .From nickel-bearing rocks we 

 may turn to chromium-bearing rocks, as exemplified by those of the 

 Great Dyke of norite of Southern Rhodesia. 1 Here, however, the 

 picrites are coarse-grained, and are not comparable in detail, either 

 texturally or mineralogically, with the Ampwihi piorite. 



Total 



100-14 



100-03 



100-46 



99-95 



99-60 



A. Picrite. Ampwihi Crossing, Mozambique (an. Harwood). 



B. Picritic basalt. Flow of 1840, Nanawale, Puna, Hawaii (an. Steiger). 



C. Diabase? Cathay Hill, Mariposa County, California, U.S.A. (an. Hillebrand) . 



D. Hornblende picrite. Ty Croes, Anglesey (an. Phillips). 



E. Peridotite? Loch Garabal, Scotland (an. Player). 



Returning to the analyses cited above, there are three further 

 relationships worthy of discussion, namely, the relation of the 

 Mg O/Fe O ratio to the percentage of alumina, the association of 

 potash with magnesia, and that of soda with iron-oxides. 



Vogt has shown that in peridotites the ratio of Mg to FeO 

 increases on an average as the percentage of alumina decreases. 2 

 For percentages of alumina between 10 and 11 the atomic ratio of 

 MgO/FeO averages 2 - 6. In the Ampwihi picrite there is consider- 

 able divergence from this value, the ratio being 4. There is a similar 

 divergence in the case of the Cathay Hill Diabase, the ratio of which 

 is 3-8. The three other rocks cited, however, give ratios that agree 

 very well with Yogt's generalization : — 



1 A. E. V. Zealley, Trans. Boy. Soc. S.A., p. 14, 1915. 



2 See A. Harker, Natural History of Igneous Bocks, p. 373, 1909. 



