part 1] AND ASSOCIATED KOCKS OF MOZAMBIQUE. 57 



No. 53, already mentioned as a coarse-grained banded gneiss, has been 

 severely crushed. The felspars are strongly cleaved, and the quartz is 

 broken into angular or slightly-rounded fragments. Fractures, cleavages, 

 and boundaries between adjacent minerals are marked by thick lines of 

 dense black dust, the nature of which is not revealed by the microscope. In 

 places, there are pockets of material entirely reduced to this same black 

 substance. 



Lines of my Ionized material, however, such as are found west of 

 Ochilesa in Benguella, have not yet been detected in Mozambique. 

 It is interesting to notice that No. 53 occurs in the Mkwassi 

 Valley, where it is met by the fault that borders the eastern walls 

 of Ribawe. Although no definite evidence was collected along 

 the Mkwassi Valley itself, it seems probable, in view of the 

 faulted boundaries of the eastern and southern walls, that the 

 northern boundary, determined by the Mkwassi Valley, is also a 

 faulted one. 



(2) Heavy Residues from the Biotite-Gneisses. 



During the course of prospecting work in Mozambique, con- 

 siderable quantities of rocks of various kinds were crushed in 

 a large iron mortar by native servants, and the crushed material 

 was afterwards panned by Mr. E. W. E. Barton (the leader of the 

 Expedition) or myself, in the hope of tracing minerals of economic 

 value. Although we were generally unsuccessful in this particular 

 quest, we never failed to obtain a residue rich in zircon, which has 

 provided valuable material for an attempt to determine the age and 

 correlation of the rocks concerned. 



From the gneissose granite of Sawa the minerals obtained were 

 magnetite and ilmenite in small quantity, the latter being more 

 abundant ; zircon in rounded grains with occasional sharply ter- 

 minated prisms, small rounded prisms of apatite less numerous 

 than zircon ; and a few examples of sphene, epidote, garnet, and 

 r utile. 



The gneissose granite of the Etipoli Hills gave a similar but 

 more abundant residue. Among the opaque minerals pyrites and 

 rnispickel were detected in small quantities, while apatite was more 

 plentiful relatively to zircon than in the Ribawe rock. 



From the biotite-gneisses the relative proportion of apatite, 

 garnet, rutile, and sphene was greater than in the granitic facies, 

 and microscopical examination shows that apatite and sphene are 

 associated more particularly with the bands rich in biotite. The 

 gneisses of the Ligonia and Lalaua rivers proved to be unusually 

 rich in pyrites and rnispickel, this being a feature already men- 

 tioned as characterizing the rocks of the Etipoli Hills which lie 

 between these two rivers. Monazite, although a common con- 

 stituent of the concentrates from many of the river-sands, has not 

 been detected in the gneisses, despite the fact that it was specially 

 looked for in view of its general widespread occurrence in granites. 1 



1 O. A. Derby, Min. Mag. vol. xi (1897) p. 304. 



