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BR. A. HOLMES 0~S THE TERTIARY 



[vol. lxxii, 



is exactly paralleled by that of the soda-amphiboles in the other 

 rocks of Group A. 



Flow-structure is of two kinds, which merge one into the other. 

 The first is determined by parallel laths of anorthoclase accom- 

 panied by radial growths of that mineral with segirine, cossyrite, 

 and ' ferrite,' occupying the wedge-shaped interstices. The second 

 type of flow-structure is that which imparts to the rock its mega- 

 scopic aspect. It is determined by the alternation of bands rich in 

 prisms of segirine, with bands containing long drawn-out vesicles 

 now occupied by opal. The opalized bands are further character- 

 ized by abundant 'ferrite.' A subsidiary type of band is mainly 

 composed of parallel laths of anorthoclase with cossyrite. Else- 

 where the laths of anorthoclase, while presenting a general 

 trachytic aspect, have crystallized in radial aggregates. The lava 

 bears a close resemblance, to the phonolitic trachyte from Lake 

 Baringo described by Dr. G. T. Prior in 1903. 1 



The radium-content of the rock was determined in 1913, and 

 proved to be 2 - 83xl0 -13 grins, per grin. The silica percentage 

 was found by the late Lieut. G. Kirby to be 64'51. 



X. Trachytoid-Phosolite. (PL XX, fig. 3.) 



The phonolites of the Sanhuti- River district (149 & 150) differ 

 from the other lavas of Mozambique in two noteworthy respects. 

 Large phenocrysts of clear glassy anorthoclase, an inch or more in 

 length, are distributed through a grey ground-mass somewhat 

 coarse in grain for a phonolite. Amygdales, averaging half an 

 inch in diameter, are also scattered through the rock, and these 

 are occupied by natrolite and calcite. The former mineral fines 

 the cavities in aggregates of white fibres, leaving the bulk of the 

 interior partly or wholly filled with calcite. 



Under the microscope, the minerals recognized and their 

 approximate percentages, determined by the Rosiwal micrometric 



method, are as follows : — 



Estimated 



Minerals. Percentage by iveight. 



Anorthoclase 33*8 



Natrolite and isotropic material . . 24'7 



Nepheline 6'6 



iEgirine and asgirine-augite 15"1 



Katoforite ~| 



Arfvedsonite V 11'5 



Barkevikite J 



Cossyrite 4*3 



Calcite '2-0 



Magnetite and ilmenite 2'0 



Apatite n. d. 



Total 100-0 



The phenocrysts of anorthoclase are prismatic, elongated 

 1 Min. Mag. vol. xiii (1903) p. 241. 



