50 DR. A. HOLMES ON THE PRE-CAMBHIAX [vol. lxxiV, 



by heating to 1200° C. a mixture of ilmenite, silica, calcium 

 sulphide, and carbon. In the present case, the interacting com- 

 pounds were probably titaniferous magnetite, quartz, and calcium 

 carbonate. A possible equation illustrating the reaction is as 

 follows : — 



FeO.Fe,'"0 3 + FeO.Ti0 2 + 4CaC0 3 + 4Si0 2 

 Magnetite Ilmenite Calcite Silica 



= Ca 3 Fe,'"(Si0 4 ) 3 + CaSiTi0 5 + 2FeO + 4C0 2 . 

 Andradite Sphene 



In the presence of water or its equivalent at high temperature, 

 some of the excess of ferrous oxide would probably be oxidized to 

 magnetite (which is present) or to ferric iron, which could then 

 make possible a further supply of andradite. In any case, some of 

 the ferrous oxide would be required for the formation of augite, 

 this mineral perhaps being produced by the interaction of ferrous 

 oxide, silica, forsterite, and calcite, or of ferrous oxide and diopside 

 (present in the limestone) : — 



2FeO 4- 7Si(X + Mg 2 SiO ; 4- 4CaC0 3 

 Ferrous Oxide Silica Forsterite Calcite 



= 4Ca(Fe, Mg)(SiO.) , + 4C0 2 . 

 Diopside 



Any ferric oxide or alumina required for the actual composition 

 of the augite would readily be forthcoming from constituents 

 present in the interacting systems, such as magnetite and spinel. 



The crystalline limestones of the district south of the Lurio, 

 near its mouth, and that occurring along the military road about 

 half-way between Itaculo and Ntia, call for no special description. 

 As represented by the specimens collected, each is a pure-white 

 saccharoidal marble with no minerals other than calcite, except for 

 a few minute specks of pyrite in the Ntia-Itaculo example. 



The crystalline limestone of the Nrassi Basin (Wo. 93) occurs 

 in a long lenticular mass, that at first sight looks like a red 

 felspathic pegmatite. The rock is a coarsely-granular dolomitic 

 marble of pink colour, and contains in places small rust}- cavities 

 representing former pyrite-crystals. As in the case of the Monapo 

 occurrence, the limestone has been cut by small intrusions of 

 microcline-granite. In the neighbourhood of the limestone, the 

 granite is characterized by numerous crystals of augite. A contact- 

 specimen (No. 94) is largely composed of the same mineral in 

 large, allotriomorphic, well-cleaved ciystals, with which are asso- 

 ciated calcite and oligoclase on the limestone side, and microcline, 

 oligoclase, and quartz on the granite side. 



In thin section the augite is seen to be bright green and free from the 

 peripheral zone of hornblende that characterizes the augite of the associated 

 granite. The maximum extinction ZAO observed is 44° ; the mean refractive 

 index, determined by the immersion method, is about 1*7 ; the maximum 

 birefringence, measured by means of a quartz wedge in a slide of known 

 thickness, is 0'025 ; and the optical character is positive. 



