446 Geology of the Ningi Hills, Northern Nigeria. 



The quartz and felspar are in some parts micrographically inter- 

 grown, while elsewhere the structure is granular-hypidiomorphic. 

 The biotite is olive-brown and contains inclusions of allanite with 

 pleochroic halves. This rock appears to be very similar to Nos. 

 108 and 110, though finer in texture. 



C.C. 4. Topaz-rock. — This is a specimen of the aplitic rock 

 associated with the tin-tungsten lodes : the actual slice was taken 

 about 1 inch from the edge of a narrow lode of this kind. It 

 consists of quartz, some highly decomposed material representing 

 felspar, topaz, and red and black patches which probably include 

 both iron oxide and wolfram. Quartz, which is very full of bubbles, 

 is the most abundant mineral, followed by topaz. Most of the 

 felspar has completely disappeared, and the specimen is an excellent 

 example of a pneumatolytic topazified rock, allied to greisen in its 

 manner of formation. Cassiterite could not be identified in the 

 slice, though the lodes carry tin. 



Conclusions. 

 The microscopic examination of these specimens confirms in all 

 respects the results arrived at by Major Williams from his field 

 observations. It is quite clear that there are here three types of 

 igneous rock, as follows : — 



1. Highly granulitized and crushed granitic gneisses, rich in 

 microcline, with little or no ferromagnesian mineral (103, 109, 111, 

 112). These appear to be essentially potash granites. 



2. Slightly crushed granites intrusive into the above, specially 

 characterized by perthite, riebeckite, and aegirine, and therefore 

 characteristically rich in soda (100, 105, 106, 113, 115). Numbers 

 108, 110, C.C. 3, C.C. 4 probably belong here. This series contains 

 accessory allanite. 



3. A still later series of soda granites, quite free from kataklastic 

 structures, with perthite, riebeckite, and segirine (124, 125, 133, 

 134, 135). These are accompanied by smaller intrusions of syenite 

 and a considerable variety of syenite dykes and other small intrusions 

 (101, 102, 120, 121, 122, 123, 129, 138, 139). This series is believed 

 to be of Mesozoic age. 



The only basic rock observed (107) is so different from the rest, 

 and so conspicuously fresh and unaltered that it may with some 

 confidence be referred to the Tertiary period. 



The first group as here defined corresponds to the older granites 

 of Falconer's classification.^ The second group are believed to be 

 also of Archaean age, although they appear to have undergone 

 considerably less dynamic metamorphism. It is as yet impossible to 

 state whether any of the hypabyssal types belong to this group. 

 The third group, which may be described collectively as the Ningi 

 series, has evidently undergone much more differentiation, since it 

 includes a large number of dykes and small masses of syenitic 



^ The Geology and Geography of Northern Nigeria, London, 1911, chap. iii. 



