part 1] AND ASSOCIATED ROCKS OF MOZAMBIQUE. 95 



argillaceous, micaceous, and ferruginous sandstones. Characteristic 

 argillaceous types have not been recognized. 



(c) Crystalline limestones, occurring as lenticular masses inter- 

 foliated with amphibolites and hornblende-gneisses, are described 

 from several localities. Contact-metamorphism by later granitic 

 intrusions has generally resulted in the formation of diopside or 

 actinolite, according to the abundance of silica ; while, in some 

 instances, garnet-scapolite rocks have developed. In one case, a 

 magnetite-pegmatite has interacted with a forsterite- marble, giving 

 a contact-rock composed of andradite, augite, sphene, and felspar. 

 It is thought that the ciystalline limestones are of exogenetic- 

 origin, and that any argillaceous rocks with which they may have 

 been originally associated have been completely assimilated by the 

 granitic magma now represented by the gneisses. Limestones 

 tend to persist — if the pressure be sufficiently high — since they are 

 not easily granitized, like argillaceous rocks, but become enclosed 

 within a blanket of hornblende-gneiss or amphibolite which may 

 preserve an internal core from the attack of igneous exudations. 



(r/) Transitions are described from coarsely-banded biotite-gneiss 

 to gneissose granite poor in biotite and nearly free from foliation ; 

 and from coarsely-banded biotite-gneiss through hornblende-gneiss 

 and amphibolite, to garnetiferous rocks and crystalline limestones. 

 It is suggested that the gneisses were produced by the concordant 

 injection of granitic magma (now represented most closely by the 

 gneissose granites of certain inselberge) into a series of pre-existing 

 sediments. Of the latter the argillaceous facies became granitized, 

 and controlled the formation of biotite-gneisses, while the cal- 

 careous or dolomitic facies formed with the granite hornblendic and 

 garnetiferous rocks. This suggestion of the composite origin of 

 the gneisses is supported by evidence based on the radioactivity 

 of the rocks. 



(e) The older rocks are riddled with enormous numbers of small 

 intrusions of granite and pegmatite, of which two series can be 

 recognized, the older being characteristically granulitic in texture, 

 while the younger is typically massive. With the older series is 

 correlated a group of augitic inclusions ranging from granite to 

 quartz-diorite. 



(f) By means of lead-ratios (Pb/U in zircon), the biotite- 

 o-neisses are correlated with the Pre-Huronian Laurentian rocks of 

 Canada ; and the granulitic granites belonging to the next period 

 of intrusion are correlated with granitic rocks in the Middle Pre- 

 Cambrian of Norway, Sweden, India, and Canada. The Pre- 

 Cambrian age of a majority of the rocks in the basal complex' of 

 Mozambique is thus established. 



(y) The inselberg landscape of Mozambique is described, and 

 it is shown that, while some of the peaks owe their survival to 

 peculiarities of structure or composition, there are others the 

 constituent rocks of which differ in no recognizable way from those 



