110 FOYE. 



The rock is greenish black in color, holocrystalline and somewhat 

 vesicular. Calcite fills the vesicles. The phenocrysts of augite are 

 in minute needles, while the feldspars (Ab4oAu6o) are in laths 4 to 6 mm. 

 in length. 



There are many irregular areas which appear to represent decom- 

 posed olivine. Chlorite, serpentine, epidote, and ([uartz are secondary 

 constituents of the ophitic ground-mass. The alteration has been 

 hastened by the action of sea-water. 



The rock has inclusions of limestone. Near these inclusions there 

 is a large amount of epidote and chlorite. The feldspar associated 

 with the contact is more acid andesine, approaching oligoclase. 



II. Rocks from Mbengha. 



A considerable collection of rocks was made both from the sea-coast 

 and from the hills of Mbengha. Eight of them were studied and it 

 was found that they were all basalts. They fall into three groups. 

 A typical specimen from each group will be described. 



1. Basalts composed chiefly of Augiie and Feldspar. It has been 

 stated in the section giving the descriptive geology of Mbengha that 

 the western side of the island is composed of flows which are little 

 dissected. These flows are characterized by their lack of olivine. 

 Specimens were collected from Ravi Ravi, from west of Wai Somo and 

 from the top of the highest hill, the Beacon. Another specimen came 

 from a dike cutting an olivine basalt at the southeastern side of the 

 island, Ndakuni. 



These rocks vary in appearance accordingly as they are holocrys- 

 talline or vitrophyric. The vitrophyric varieties are dark gray to 

 black in color while the holocrystalline, diabasic varieties are greenish 

 black. 



One of the vitrophyric varieties has augite ((Hallage) and feldspar 

 phenocrysts, in crystals 5X3 mm. in size, abundantly scattered 

 through a glassy paste. Another is vesicular and the phenocrysts 

 are so minute in size that th(\v are indistinct in the slate-gray ground- 

 mass. 



The holocrystalline variety is very fine-grained, with minute needles 

 of feldspar showing fluidal arrangement about an occasional augite 

 phenocryst, 3 to 4 mm. in diameter. 



The feldspar is almost always zoned and varies from acid to basic 

 lal)ra(l()ritc (Abr,(iAii:,n to AImsAuts). The feldspar of the ground-mass 

 is usually a medium labradorite (Ab4oAn6o). 



