Geology of the Ningi Hills, Northern Nigeria. 445 



and micrographic, while in other parts the minerals show the relations 

 characteristic of a dolerite. The specimen also contains a good many- 

 pebbles of quartz, with corrosion borders, obviously xenolithic, and 

 it is probable that the whole thing is a mixture rock, composed 

 of alkali-granitic and alkali-doleritic materials. Further material 

 is necessary to elucidate this point. 



133 and 134. Riebeckite-granite. — A granitic rock of coarse texture, 

 consisting exclusively of quartz, white perthite, and riebeckite. 

 Quartz is very abundant and shows a slight tendency to graphic 

 intergrowth with the felspar. The riebeckite is of a deep prussian 

 blue colour, visible even in the hand-specimen ; it forms irregular 

 patches, and occasionally long idiomorphic prisms. The only 

 accessory observed is a small amount of deep purple fluorspar, 

 occasionally visible even in the hand-specimen, but chiefly 

 microscopic. 



135. Riebecldte-'pegmatite. — A small hand specimen associated 

 with the two foregoing, of fairly coarse pegmatitic character, consists 

 of quartz, flesh-red felspar, and large prisms of riebeckite up to two 

 inches in length. 



137. Biotite-aplite Vein. — This specimen consists of a perthite- 

 granite of fairly coarse texture penetrated by a vein of aplitic 

 character, with red-brown biotite. 



138. Augite-porphyry. — Phenocrysts of pink and occasionally 

 white felspar in a matrix of quartz, felspar and augite, now repre- 

 sented mainly by some chloritic mineral. The large white felspars, 

 which appear to be albite, have a border of pink orthoclase, and 

 the pink felspars are orthoclase so far as they can be determined : 

 all are considerably decomposed. The groundmass is rather fine 

 in texture, mainly orthoclase in irregular patches with pale green 

 augite, chlorite and a little interstitial quartz, with some opaque iron- 

 ore. This is the hypabyssal facies of a syenitic rock, with dominant 

 potash. 



139. Mgirine-rhyolite. — A greyish-green rock with very con- 

 spicuous flow-structure. It contains phenocrysts of quartz and 

 perthite felspar in a groundmass of variable texture : some bands 

 show a microgranitic structure, while others are finely micro- 

 crystalline. The minerals of the groundmass are quartz, felspar 

 and aegirine, the latter in small shapeless or elongated grains, with 

 an occasional larger prismatic form : the latter occur chiefly in the 

 coarser bands. No other ferromagnesian minerals were observed. 



140. Graphic Granite. — The minerals of this specimen are too 

 much decomposed for certain determination. The felspar was 

 probably orthoclase, since no indications of polysynthetic twinning 

 could be seen. The only noticeable feature is the well-marked 

 micrographic structure. There are a few large zircons and a little 

 magnetite. 



C.C. 3. Biotite-granulite. — A rock of granulitic or aplitic 

 structure, consisting of quartz, microcline, orthoclase, and biotite. 



