44 DR. A. HOLMES ON THE PRE -CAMBRIAN [vol. lxxiv, 



III. The Crystalline Schists. 



About 4 miles north of Mitikiti, in the banks o£ a stream that 

 drains into the Mitikiti Iliver, bands of gneissose rock that 

 are undoubtedly of sedimentary origin are exposed. The folia- 

 tion is nearly due north-east and south-west, and is parallel to 

 that of the biotite-gneisses with which the sediment-gneisses are 

 associated. In hand-specimens the rock is seen to be composed 

 of irregular interlocking bands that are rich in quartz and biotite 

 alternately. For this reason, the formation has a drawn-out and 

 coarsely-speckled appearance in the field, and is structurally a 

 sediment-gneiss rather than a mica-schist. 



No. 186 1 . — Under the microscope, the rock still retains in part the aspect of 

 an arenaceous sediment, the clastic structure being astonishingly well preserved. 

 Angular fragments of quartz free from any traces of distortion or recrystal- 

 lization, and subangular fragments of felspar (chiefly orthoclase or microcline, 

 with occasional examples of oligoclase) which have suffered little, if any, 

 distortion, are the most abundant minerals present. Around the borders of 

 these, shreds of biotite have developed, maintaining on an average a parallel 

 orientation which alone has determined the foliation. In places there are 

 similar streaks of muscovite ; while grains of epidote and garnet and specks of 

 iron-ores are distributed sparsely among the other minerals (PI. VIII, fig. 1). 

 Quartz and felspar exhibit no tendency towards elongation in the direction of 

 foliation, and are remarkably uniform in their dimensions, the average diameter 

 being about 0"15 millimetre. 



JVos. 181-83. — In the Monapo bed (between Mitikiti and Muri- 

 um tigri), a number of pebbles of quartz-magnetite-granulites and 

 schists were collected by Mr. Wayland ; but, despite a close search 

 for their parent rocks, nothing at all resembling them was found 

 in situ. 



No. 183 is a finely -foliated dark-grey rock, which in thin section is seen 

 to consist mainly of a fine-grained mosaic of recrystallized quartz. A few 

 grains of orthoclase, with inclusions of apatite, are also present. Both the 

 quartz- and felspar-grains are roughly equi dimensional, and it is to the 

 arrangement of the other minerals that the foliation is due. Irregular small 

 masses of magnetite are very numerous throughout the rock, and along certain 

 bands rounded grains of epidote, badly-terminated crystals of kyanite, and 

 elongated prisms of andalusite are also abundant. 



In No. 181 recrystallization appears to have proceeded much farther, for 

 the quartz -felspar mosaic is made up of individuals having average diameters 

 of 0*3 millimetre, instead of 0'15 mm. as in the preceding types. The mineral 

 composition is the same as that of No. 183 ; but, as in this case the crystals 

 of magnetite are drawn out into long lath-shaped masses which are not con- 

 spicuously parallel, the rock is granulitic rather than schistose. 



No. 182 is even coarser in texture, irregular patches of quartz reaching 1/0 

 or 1*2 mm. in their largest dimensions. Very little felspar is present; but 

 magnetite, in drawn-out crystals and short rounded stumpy grains, makes up 

 about 20 per cent, of the rock. Andalusite and kyanite are absent. (See 

 PI. VIII, fig. 2.) 



1 The numbers refer to specimens in the collection of the Mozambique 

 rocks studied by the Author. That collection has now been presented to the 

 Mineralogical Department of the British Museum (Natural History). 



