WORKING GROUP 2
STEPHENS
101
developed in the north, where porphyroblastic-migmatitic-biotite gneiss pre
dominates. The top 12,000 feet of the metasedimentary series comprises inter-
bedded dolomitic marble and gneiss preserved in the centre of a large and
complex syncline, the axial plane of which is itself synclinally folded.
It was not possible to distinguish on air photographs between the hornblende-,
biotite-, and quartzo-feldspathic gneisses, which formed the main group of
paragneisses, because they were generally closely banded and intimately asso
ciated. Field evidence indicated two areas where biotite gneiss predominated,
and these were delineated photogeologically.
The interbedded dolomitic marble and gneiss, which together form the
Crystalline Limestone Group of Mahenge, were more susceptible to inter
pretation. After a few check field traverses across the strike, it was possible
to delineate the dolomite from the interbedded gneisses successfully on photo
graphs mainly on the basis of differences in topographic expression. The marble
is less resistant to erosion than the gneiss which forms parallel ridges while the
marble occupies the intervening valleys. Pinnacles of dolomite, as much as 50
feet high, are abundant in the broader valleys and these can be observed on
photographs under high power. Lenticular bodies of quartzite occurring
within this series formed ridges which could not be distinguished photogeol
ogically from the ridges of paragneiss.
Outcrops of orthogneiss are uncommon in the Mahenge area. A few small
intrusions of gabbroic and ultrabasic rock have been altered by regional meta
morphism to foliated meta-gabbro, garnetiferous-anorthositic meta-gabbro,
hypersthene amphibolite, garnet hornblendite, talc schist, and tremolite-
chlorite schist. The boundaries of the smaller intrusions (approximately 100 to
200 yards across) were not clear on the air photographs, although the ortho
gneiss bodies usually formed topographic features distinct enough to be recog
nised on air photographs as unusual outcrops worthy of examination in the
field. The largest intrusion (approximately 1 mile by 2 miles) formed a steep
hill, and it was possible to delineate the boundary of the orthogneiss with fair
accuracy.
A suite of vertical dykes altered to quartzo-feldspathic gneiss formed positive
linear features and could be identified photogeologically with certainty. Ver
tical, post-basement dykes of foliated kersantite, metadolerite, and minette,
are intruded into the paragneisses along major joints which can be traced
photogeologically as continuous lineaments for as much as ten miles. The
dykes do not form positive features because they are less resistant than the
gneisses, but the ability to trace the major joints on photographs was itself of
great value in locating the dykes in the field.
The main contribution of photogeology in the Mahenge area, Tanganyika,
was in the interpretation and mapping of the refolded syncline within the
metamorphic series. The identification of individual metamorphic rocks pre
sented great difficulty and was not possible by photogeological methods alone,
and a number of field traverses, selected after a detailed examination and
interpretation of the photographs, were necessary before any delineation of
