THE ARCHiEAN FORMATION OF THE ABUKUMA PLATEAU. 251 



rock. Tlie other extreme is a rather dark, imperfectly schistose, band- 

 ed rock with red crystals of garnet sprinkled all through it. The 

 microscopical components are the same in both varieties. The main 

 mass is made up of quartz-grains with the tombac-brown biotite 

 between them. The biotite is found in the substance of the quartz as 

 enclosures, but the grains of quartz are comparatively free from 

 liquid and gas inclusions. Muscovite is here wanting. 



Accessories are sillimanite in fine radiating tufts (Fig. 3, PI. 

 XXVI), prisms of zircon, and clumps of iron glance. Garnet 

 is a never-failing but accidental component. In the iirst. the quart- 

 zose, variety of the schist the garnet «jccurs in minute, almost colour- 

 less dodecahedra, while in the second, the Ijasic, schist it is somewhat 

 large, granular, and cherry-red. The colour of the garnet seems to Jiave 

 some connection with the quantity of biotite present in the rock ; for, in 

 the dark varietv with abundance of the black mica, tlie "•arnet is red. 

 This mineral is, extremely rich in enclosures of quartz, and in nega- 

 tive crystals filled with air, so as to make it appear like some crystals 

 of staurolite. 



A d) Hornblende- Biotite- Schist. 



This is "exactly the same in its external appearance as the gneiss- 

 mica-schist, already described, differing, however, from it by the 

 presence of hornblende as an essential component, Tlie rock is grey, 

 and splits easily into an even tabular plate. The feldspars, both the 

 monoclinic and triclinic, make iqj the greater part of the mass, while 

 the amount of quartz decreases as they increase. The deep-brown 

 biotite and the bottle-green hornblende are intimately plaited toge- 

 ther, showing none of their crystallographic outlines. The zircon 

 and apatite, both in the form of grains, are especially aijundant in this 

 rock. Locality : Xishida, east of Tanagura, wliere the rock forms a 



