Geology of the Ningi Hills, Northern Nigeria. 441 



tributary). Near to Zelau the streams are markedly stanniferous, 

 especially those draining south and south-east to the Delimi valley. 

 From the Jengre Hills several Delimi tributaries, especially the 

 Buji and Zora, carry very considerable amounts of tin in the wash. 



The Ningi granites yield tin, and several streams in the western 

 end of these hills are being worked on a very small scale. Small 

 quantities of tin, never in payable amounts, were found both in the 

 Kila and the Pagam Hills. In these areas, however, the absence of 

 defined watercourses militates against the economic concentration 

 of the shed tin. No tin was ever found in streams draining only from 

 the gneisses. For example, in the rivers of the Yarde Kerri Group 

 Mines, the Gongome, Kogo, Kuskerri, and Tipchi River and river- 

 flats which drain the Sabon-Garri type of granite produce tin in 

 economic quanity, but the adjacent left fork of the Koteni, which 

 drains a gneissose area, is absolutely barren of tin. This was also 

 observed in the streams draining a large gneissose area between 

 Begindi and Rahama. 



If a generalization may be permitted at this stage I would say 

 that tin is only found in granites that contain riebeckite — and this 

 view derives further support from a consideration of the Mesozoic, 

 tin-bearing granites of the Ropp-Monguna-Exlands areas in the 

 southern portion of the Bauchi Plateau. On the other hand, it 

 occurs equally in riebeckite granite of both Mesozoic and Archaean 

 age ; indeed, as far as the area traversed is concerned, it is far more 

 abundant in the Archaean granites than in those of the Ningi type. 



PETROGRAPHICAL APPENDIX. 



The following notes are brief petrographical descriptions of the 

 rock-specimens collected by Major Williams during the traverse 

 described in the preceding paper. Most of the specimens have 

 been sliced, but in a few instances weathering had advanced too 

 far for this. These notes have purposely been made very short 

 as it is hoped to obtain further specimens of the more interesting 

 types and to publish a full account of the petrography of this area 

 on a subsequent occasion. The nomenclature here adopted must be 

 considered as provisional, since further information is needed as to 

 the field-relations of some of the types collected, and no attempt 

 has been made to institute comparisons with similar rocks elsewhere. 



Description of Specimens. 



100. Rieheckite-biotite-granite. — This is a rock of very coarse 

 texture, with crystals of felspar up to an inch in length, quartz, 

 and large patches of dark-coloured minerals. The largest felspars 

 are perthite, the rest mainly albite, with a little orthoclase and 

 microcline. Quartz is fairly abundant, both as large crystals and 

 as rounded blebs in the felspar, with a little micropegmatite. The 

 ferromagnesian minerals are riebeckite and deep brown biotite in 



