Note on the Rocks of Darfur. 213 
mediate between olivine-basalts and kenytes,! and which Jensen 
_ described later as basic kenytes or plagioclase-kenytes.” 
The hand-specimen shows small phenocrysts of felspar, averaging 
0°6 mm. in length, in a dull glassy base. Under the microscope the 
eroundmass is seen to be very dark, almost opaque, with scattered 
microlites of felspar giving straight extinction, possibly anorthoclase, 
but too thin te be determined with certainty. The other crystals in 
the glassy groundmass are olivine or its deep-red alteration product, 
one or two ‘pale yellow crystals of augite, 1 mm. in diameter, several 
opaque black lozenge-shaped pseudomorphs possibly after horn- 
blende, a few smaller augites of a pale-green colour, and one or 
two rounded flakes of dark-brown biotite. The felspar phenocrysts 
are of two kinds, basic andesine, near Ab, An,, and anorthoclase, 
the latter being present in slightly smaller amount than the andesine. 
The anorthoclase forms tabular Carlsbad twins, of which the ends are 
rounded and the sides are frequently embayed by patches of ground- 
mass. It occasionally encloses olivine or its alteration products. 
The andesine tends to develop euhedral outlines and contains 
inclusions of groundmass, and in one case small crystals of apatite. 
In addition to the phenocrysts of felspar the section shows five or 
six rounded crystals of olivine. 
MUGEARITE. 
A specimen chipped from a boulder in the Wadi Ghindi 
at about 7,500 feet (22) is a fine-grained blue-grey rock with 
trachytic texture mainly composed of lath-shaped crystals 
(0°2mm. X 0°02 mm.) of oligoclase, associated with orthoclase, 
granular augite, and magnetite, with a few small phenocrysts 
( < 0-4 mm. diameter) of olivine and magnetite and occasional brown 
crystals of apatite. This compares so closely with the mugearites, 
described first by Harker? and later by others, that one has no 
hesitation in assigning it to this species. 
By the courtesy of Dr. H. H: Thomas I have been able to examine 
Dr. Harker’s type rock from Druim na Criche, near Portree, Skye, 
and the other mugearites described from time to time by members 
of H.M. Geol. Survey. 
Compared with Harker’s type, the Wadi Ghindi rock is much 
poorer in olivine. Another point of difference is the presence of 
small phenocrysts of felspar, giving symmetrical extinctions of up 
to 11°, and consisting of oligoclase slightly more basic than that of the 
small laths, and sometimes showing a slightly more acid outer 
zone. The groundmass contains more augite and less magnetite 
‘than the type rock, and the augite is granular and not ophitic. The 
" G. T. Prior, ‘‘ National Antarctic Expedition, 1901-4’’: Nat. Hist., 
vol. : , 1907, p. 106. 
2H. if: Jensen, “ British Antarctic Expedition, 1907-9’’: Geology, vol. ii, 
1916, pp. 107-9. 
oA. Harker, ‘‘ The Tertiary Igneous Rocks of Skye’’: Mem. Geol. Surv. 
U.K., 1904, p. 264. 
