part 1] A^D ASSOCIATED ROCKS OF MOZAMBIQUE. 75 



examination of the analysed material revealed traces of both 

 quartz and rutile, and the presence of about 1*8 per cent, and 

 1*3 per cent, respectively of these impurities would suffice to 

 reduce the silica to the orthosilicate proportion, and to explain the 

 amount of titanium dioxide recorded in the analysis. The excess 

 of water is not so easily disposed of. If it be assumed that, in 

 place of aluminium, the group (Al OH) is present in considerable 

 proportion, then the muscovite is calculated to have the following 

 approximate composition : — 



J 55 per cent, of R^Al.^SiO,), 



1 45 per cent, of R' ;i (A10H) s Al(SiOJ :i ; 



where R' is K, Na, and H; Al partly replaced by Fe"'; (AlOH) partly 

 replaced by Ca, Mg, and Fe" ; and (OH) partly replaced by F. 



(5) Granulitic Norite rich in Sphene. 



A dark-grey finely-granulitic rock (No. 37) occurs as sill-like 

 masses penetrating biotite-gneiss in the bed of the Namieta River, 

 south of Peone Hill (Mwipwi Mountains). Under the microscope 

 the rock is found to be composed of labradorite, pale greenish-grey 

 augite. nearly colourless enstatite, and clo've-brown sphene, with 

 pyrite and magnetite as accessories (PI. X, fig. 2). The texture 

 is thoroughly granulitic and the range in dimensions is low, the 

 average grain being about 2 mm. The pyroxenes are usually above 

 average size, and by aggregation they form still larger groups ; the 

 rounded and elongated crystals of sphene are, on the contrary, 

 below the average size. Like the rock as a whole, the felspar is 

 perfectly fresh, and its optical properties indicate that its com- 

 position is approximately Ab 45 An, 5 . A single small individual of 

 quartz Avas seen in one section only. 



The mineral composition by volume, determined by the Eosiwal 

 method, is as follows : — 



Quartz trace 



Labradorite 43 Specific gravity : 



Pyroxenes 47 Calculated = 3*07 



Sphene 6 Measured = 3*04 



Apatite 1 



Pyrite 2 Radium-content : 



Magnetite 1 in grins.- 12 per grm. 



of rock 1 = 0-54 



100 



This rock is the only one so far found in Mozambique that 

 resembles in any way the rocks of the charnockite series of India, 2 

 or of the similar series which have been discovered in Ceylon, 3 the 

 Ivory Coast, 4 and Benguella. 5 



1 A. Holmes, ' The Age of the Earth' 1913, Appendix A, p. 183. 

 - T. H. Holland, Mem. Geol. Surv. India, vol. xxviii (1900) p. 124. 

 :i A. K. Coomaraswamy, Q. J. G. S. vol. lviii (1902) p. 401. 



4 A. Lacroix, C.R. Acad. Sci. Paris, vol. cl (1910) p. 18. 



5 G. W. Tyrrell, Trans. Roy. Soc. Edin. vol. li, pt. 3 (1916) p. 539. 



