

Geological Society, 527 



expedition was organized by the Memba Minerals Ltd., and during 

 the second season, Mr. E. J. Wayland, Mr. D. A. Wray, and the 

 author visited the country as geologists to the Company. 



With the exception of a coastal belt of Cretaceous and Tertiary 

 sediments, flanked on the west by later Tertiary volcanic rocks, the 

 whole territory consists of a complex of gneisses and other foliated 

 rocks, intruded upon by granites belonging to at least two different 

 periods. From Fernao Vellosa Harbour to Mokambo Bay the 

 junction of the sedimentary formations with the crystalline com- 

 plex is a faulted one, and the volcanic rocks, now greatly dissected 

 by erosion, are distributed on each side of the fault. The lavas 

 are of post-Oligocene age, and are clearly the result of fissure- 

 eruptions, the feeding channels being exposed as numerous small 

 dykes that penetrate the underlying rocks. 



Throughout the area the prevailing lavas are amygdaloidal 

 basalts, in which the chief amygdale minerals are chlorite, heulan- 

 dite, and fonns of silica. An andesite dyke of later date occurs 

 near the Monapo River. In the north, near the Sanhuti River, 

 picrite-basalt, basalt, phonolite, and solvsbergite have been found, 

 and related lavas occurring elsewhere in the area are tephritic 

 pumice and aegirine-trachyte. Thus, within the limits of a 

 small volcanic field, series of both ' alkali ' and ' calc-alkali ' 

 types of lava occur together. The ' alkali ' series can be closely 

 matched by the lavas of Abyssinia, British East Africa, Reunion, 

 and Tenerife. The amygdaloidal basalts of the ' calc-alkali ' series 

 are similar to those of the Deccan, Arabia, and East Africa, and 

 also to those (of late Karroo age) occurring in South Africa and 

 Central Africa. 



In all, ten analyses have been made, and the variation-diagrams 

 constructed from them support the view that each of the series 

 was evolved by a process of differentiation acting on a parent 

 magma. From the composition of the amygdale minerals (which 

 are referred to the closing phase of lava-consolidation), it is 

 deduced that the parent magma of the ' alkali ' series was rich in 

 carbon dioxide, and undersaturated in silica ; whereas that of the 

 ' calc-alkali ' series was rich in water, and oversaturated in silica. 

 The radioactivity of the lavas indicates that the depth from which 

 the parent magma came was probably between 33 and 44 miles 

 from the earth's surface. The boundary-fault along which the 

 lavas are aligned seems to mark a zone where pressure was relieved 

 to an extent and depth sufficient to promote fusion. 



Dr. A. Stkahaj*, F.R.S., exhibited cores from borings 

 in Kent, showing pebbles of coal embedded in Coal- 

 Measure sandstones. With the coal-pebbles occurred a few 

 partly-rounded fragments of chert, and in one of these radiolaria 

 bad been identified by Dr. G-. J. Hinde. The chert resembled 

 that which had been described from Lower Carboniferous rocks 

 elsewhere. Its occurrence suggested that the sequence of strata had 

 been similar in South Wales and Kent, and, taken in connexion 



