﻿238 



DR. A. HOLMES ON THE TERTIARY 



[vol. lxxii r 



Owing to the smallness of the grains and the lack of cleavage, 

 the optical orientation of cossyrite could not be determined. The 

 refractive index is near T66, but slightly higher. Pleochroism is 

 in tints of deep brown, reddish-brown, and lighter brown, and can 

 only be observed in a very thin section. 



The first minerals to crystallize out were magnetite, ilmenite, 

 and apatite, followed by phenocrysts of anorthoclase and cossyrite. 

 After partial resorption of the phenocrysts, during which the 

 anorthoclase suffered peripheral alteration to kaolin, the crystalli- 

 zation of the groundmass began. Nepheline and anorthoclase 

 were followed in order by cossyrite, segirine, katoforite, and 

 arfvedsonite. Finally, natrolite, a little calcite, and an obscure 

 isotropic substance filled up the still-unoccupied spaces ; also 

 natrolite and calcite were deposited in the vesicles. 



Chemical Composition. 



An analysis of fresh portions of No. 149, free from visible 

 amygdales, gave the following results : — 



Molecular Mineral Composition 



Percentages. Proportions. (Norm). 



Si0 2 56-34 -939 Orthoclase 27-80 ) 



A1,0 3 17-86 -175 Albite 46 11 | Salic 



Fe">0 3 4-43 -028 Nepheline 8-52 [ =84-38. 



FeO 3-81 -053 Anorthite 1-95 1 



MgO 0-52 -013 



CaO 1-97 -036 Diopside 3-07^ 



Na.,0 7-33 -118 Olivine 1-89 



K.,0 4-72 -050 Magnetite 6-50 }> 



H"0+ ... 1-22 ... Ilmenite 1"22 j 



H.^O- ... 0-82 ... Calcite 1-60 J 



C0 2 0-71 -016 



TiO\ 0-67 -008 98-66 



P 2 Os tr. 



MnO 0-28 -004 Water 2-04 



Feinic 

 = 14-28. 



Total 100-68 Total 100-70 



Class II Dosalane. 



Ra = 4 - 10xl0~ 12 grms. per grm. Order 5 Germanare. 



Specific gravity = 2-4 7. Rang 1 Umptekase. 



Subrang 4 ... TJmptekose. 



Analyses of similar rocks from neighbouring regions are quoted, 

 below for comparison. The Mozambique example is almost 

 identical with the phonolite from Mount Kenya analysed by 

 Dr. Gr. T. Prior. The similarity extends equally to the mineral 

 composition. Dr. Prior has kindly allowed me to examine the 

 phonolites from British East Africa in the Natural History 

 Museum, and, except in texture, they bear a striking resemblance 

 to the phonolite from the Sanhuti River. The latter rock, like 

 the associated solvsbergite, is unusually rich in iron-oxides, this- 

 being due to the presence of magnetite as an accessory. 



