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Transactions of the Symposium on Photo Interpretation

the slight increase in hardness of the leucocratic rock and its paler tones.
Quartzo-feldspathic gneiss form narrow continuous ridges, generally a few
hundred yards wide, within this group, which can be traced round the outcrops
of the tight isoclinal folds. These resistant quartzofeldspathic ridges are of great
assistance in the photogeological interpretation of the complex structures within
this group. (Fig. 2).
The third group to be identified photogeologically consists mainly of horn
blende gneiss and forms fairly rugged dissected country with a dense vege
tation cover. The delineation of the group depended, however, to a large
extent on field observations.
The metasediments change southwards along their strike from wellfoliated
gneisses in the north to finer-grained rocks with granulitic texture in the south.
This is a gradual change which cannot be observed photogeologically.
Perthitization has occurred in a number of localities resulting in the formation
of a coarse, strongly-foliated, gneissose rock which in most places forms steep
hills because of its resistant nature. The most advanced stage of perthitization
in this area has produced augenperthite gneiss in which abundant porphyro-
blasts of perthite can be seen in the hand specimen.
The bodies of perthite gneiss could be delineated successfully on photographs.
(Fig. 3). The smaller masses form bare steep-sided crags, while the larger masses
(approximately 9 miles by 3 miles) generally produce high mountainous coun
try with sharp boundaries. The strike of the perthite gneiss is concordant with
Fig. 3.
Stereopair: Augen-perthite gneiss, Lilongwe-Salima Area, Nyasaland
Shows the southern margin of a large body of perthitized gneiss. The rock is a coarse-grained,
hornblende gneiss containing abundant augen of perthite and is well-jointed. The lower
ground consists of finer-grained gneiss while ferruginous gneiss forms the paler-toned hills.
(Reproduced by permission of the British Air Ministry, British Crown Copyrigt reserved).