214 Note on the Rocks of Darfur. 
felspar laths are slightly larger. They give straight extinction 
and are oligoclase; orthoclase is present, but is less conspicuous 
than in the type. Apatite does not occur as abundant small crystals, 
but is represented by a few small prisms, coloured brown by lines 
of small inclusions and showing pleochroism: X = dark smoky 
brown, Z = yellowish-brown. Precisely similar apatites are found 
in mugearites from Kast Lothian and from Pembrokeshire The 
brown hornblende and biotite present in the mugearite of Skye and 
of other localities is absent. The Wadi Ghindi rock has a density 
of about 2°6, and is probably more acid than Harker’s type. 
With the exception of the mugearite and the andesine-bearing 
kenyte, the above rocks compare very closely with types 
described from the Kast African volcanoes. It is interesting to note 
the occurrence of rocks with similar affinities among those found by 
Lieut.-Col. Tilho at the volcano of Emi Koussi, in Tibesti, 600 miles 
north-west of Jebel Marra.? Professor Lacroix has not given detailed 
descriptions or figures of these rocks, but the catophorite-bearing 
trachyte mentioned ® will doubtless be found to compare closely 
with the soda-trachytes or quartz-bearing soda-trachytes above 
described. The descriptions of the crater of Emi Koussi show that 
in form it closely resembles the crater oi Dereiba.* 
[Captain Lynes has now returned to Darfur, and intends making 
a long stay in Jebel Marra to study its natural history. It is hoped 
that after his return it will be possible to furnish further details of 
the geology and petrology of the district. In the meantime these 
preliminary notes may serve to indicate the nature of the rocks 
which occur in this part of Africa, and to show what interest may 
be attached to collections from remote parts of the world of fresh 
rocks taken in situ, carefully located and carefully labelled.— 
Wie 253] 
APPENDIX. 
Report of the examination of a sample of water sent by Capt. Hubert 
Lynes, R.N., and recewved on June 7, 1920. 
Sample No. 5728, labelled : “‘ Water and a little green scum from 
‘Salt Lake’, Jebel Marra Crater ; altitude 5,700 feet.” 
The green scum referred to had disappeared, but there was a small 
amount of dark green sediment, showing organic structure, at the 
bottom of the bottle. 
Sample No. 5729, labelled: ‘“‘ Water from ‘ Fresh Lake’, Jebel 
Marra Crater ; altitude 7,500 feet.” 
Analysis gave the following results, which are expressed as parts 
per 100,000 parts of water :— 
1 H. H. Thomas, ‘‘ The Skomer Volcanic Series’: Quart. Journ. Geol. Soc., 
vol. Ixvii, 1911, p. 197, fig. 9b. 
2 A. Lacroix, O.R. Acad. Sci. Paris, vol. clxix, 1919, pp. 401-7. 
3 Tbid., p. 404. 
4 A. Lacroix, O.R. Acad. Sci. Paris, vol. clxviii, 1919, pp. 1237-40, and 
vol. clxix, p. 403. 
