Geology of the Ningi Hills, Northern Nigeria. 443 



with accessory apatite and allanite, and a trace of purple fluorite. 

 Although not apparently gneissose to the naked eye, the microscope 

 shows that these rocks have been to some extent crushed and 

 granulitized, though not so much so as the thoroughly gneissose types. 

 109. Granite or gneiss. — This appears to have been originally 

 a gneissose granite with micropegmatite and without ferromagnesian 

 minerals, but it is now so much decomposed that the original 

 character of the felspar is indeterminable. 



111. Sheared gneiss. — This rock consists of quartz and orthoclase 

 without any original coloured minerals, but there is a little 

 secondary epidote. It is very much sheared and granulitized. 



112. Biotite-granite. — A rock of coarse texture and somewhat 

 granulitized, consisting of quartz, microcline-microperthite, and 

 brown bioCite. It presents no features of particular interest. 



113. Porphyritic biotite-granite. — The felspar of this rock is so 

 much decomposed that its original character is uncertain, but it 

 was probably perthite. The other minerals are quartz and brown 

 biotite, and the structure of the groundmass is microgranitic. 



114. Biotite-granite. — This specimen seems to show the contact 

 between the granulitized gneissose granites of the ancient complex 

 characterized by olive-brown biotite and the riebeckite-biotite 

 granites of the later Ningi series. The two are, however, so 

 inextricably intermingled along the margin that no clear description 

 can be given. In this slice only a small trace of riebeckite can be 

 identified, and the greater part of the specimen seems to consist of 

 the older rock, more or less shattered by the later intrusion. 



115. Biotite-rieheckite-granite. — A very much decomposed rock, 

 composed of quartz and perthitic felspar in graphic intergrowth, 

 with chestnut-brown biotite and riebeckite, and much iron oxide. 

 There is no sign of crushing, and this rock evidently belongs to the 

 younger series. 



117. Quartz-jjorphyry. — A typical example of an acid hypabyssal 

 rock with phenocrysts of quartz and twinned orthoclase ; some 

 phenocrysts consist of micropegmatite, and the quartzites show very 

 good dihexagonal forms. The finely microcrystalline groundmass 

 consists of quartz, felspar, and brown iron oxide. The quartz and 

 felspar show micropoecilitic relations, and flow-banding is well 

 marked. No ferromagnesian minerals remain, but some patches 

 of yellow iron oxide may represent phenocrysts of biotite. 



120. Quartz- syenite. — Very abundant orthoclase in large crystals 

 with some plagioclase (albite and oligoclase), and a small amount of 

 interstitial quartz. The ferromagnesian minerals are rather decom- 

 posed ; mainly hornblende, but a little biotite was probably originally 

 present. The hornblende is of a blue-green soda-bearing variety 

 near to riebeckite, with strong pleochroism (greenish-blue, olive- 

 brown, yellowish). There is a fair amount of accessory apatite and 

 a little magnetite. 



121. Orthophyre. — A porphyritic rock with phenocrysts of pink 



