416 A. Holmes i^- D. A. Wraij — Geology of 2Iozamhique. 



Of great interest are the lavas so well developed along the coastal 

 strip and hitherto not described or referred to elsewhere. Basaltic 

 lavas were extruded over a large area, but denudation has now 

 reduced them to scattered remnants. 



Amygdaloidal basalts, clearly the result of fissure-eruptions, are 

 found in a zone never far from the coast, and stretching northwards 

 from Mokambo Bay for some 20 miles. Associated with them, and 

 representing the ducts up which the molten magma found its way to 

 the surface, are numerous dykes and sills of dolerite. Pebbles of 

 agate and jasper, derived from the cavities in the basalt, are found 

 far to the west and north of the present exposuj-es, and indicate 

 roughly the former extensions of the flows. Besides quartz and 

 chalcedony, zeolites of various types are found filling the amygdiiles. 

 The basalt and dolerite differ only in the coarser crystallization of the 

 latter and its relative freedom tVom vesicles. Olivine appears to be 

 entirely absent, its place being taken by the orthorhombic pyroxenes. 

 In this respect the rocks differ from other East African basalts, for in 

 the latter olivine is generally a noteworthy constituent. The flows lie 

 indiscriminately on the Archfean gneisses, and on the Cretaceous and 

 Tertiary sediments, while the dykes and sills intrude through the same 

 formations. Where they are in contact with the Cretaceous limestone, 

 this has been transformed into a crystalline aggregate within a few 

 inches of the contact. 



Along the coast and for some distance inland are found numerous 

 raised beaches lying at heights up to 25 feet, while the whole coast is 

 fringed with coral reefs. These deposits afford further proof of the 

 recent uplift of the East African coast, evidences of which have 

 also been met with both to the south and north. Another indication 

 is found in the rejuvenation of the streams which, thus endowed 

 with greater energy, have been enabled to cut gullies in the gravels 

 and sands of an earlier epoch. The superficial deposits which are 

 now being dissected are the products of disintegration of the older 

 rocks. These we found to consist in general of two types : the first, 

 enveloping the hills, and due in the main to the disintegration of 

 rock in situ, and the second found in the plains and clearly showing 

 evidence of having been transported by water- action. 



The decomposition of rocks in situ gives rise to two widely different 

 varieties of residual deposits. The one consists of kaolin, the other 

 of laterite, and in most cases angular particles of quartz are present. 

 The former type of alteration calls for little comment, but it maybe 

 observed that the gradual transition of hard rock into soft china-clay 

 could easily be traced in the field, suggesting that surface action 

 alone had brought about the change. 



A quartzose laterite is the more usual superficial formation. In 

 general it forms hard, brown, slaggy-looking masses, with concretions 

 of iron-ore containing numerous angular fragments of quartz. The 

 rock is penetrated by cavities which may be partly occupied by 

 a powdery ferruginous earth. The laterites usually occur in bands 

 running parallel to the strike of the gneiss, and often these lie near 

 the crest of low ridges which follow the same direction. They 

 also occur near the sides of small watercourses, of which the majority 



