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SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
therefore an example in which smaller scale photography results in a clearer 
and more reliable interpretation than does larger scale [see Hemphill, 1958]. 
The bands of the permeation gneiss are believed to owe their existence to the 
pre-granitization relic bedding and foliation planes within the original meta 
sediments from which they were formed, and thus to give an indication of the 
pre-granitization structure of the area. 
Although one can provide no proof of the origin of the bands in the per 
meation gneisses, the study of larger and larger areas of them does provide 
increasingly strong circumstantial evidence of their origin. The writer’s photo- 
geological interpretation of the Yelwa Sheet of Northern Nigeria contained 
some 200 square miles (518 sq.km) of permeation gneiss whose lineaments 
suggested relic bedding rather than say, flow banding within a granitic in 
trusion. The similarity in appearance between the lineaments of the permeation 
gneiss west of the River Niger and those of the metasediments east of the river 
was noteworthy. 
Once the geologist has satisfied himself by field work that the “flowing” 
appearance is characteristic of soil-covered permeation gneiss, and that the 
“flowing” lineaments represent the relic bedding of the permeation gneiss, 
then he is able to map the permeation gneiss and the relic bedding structures 
within it, both quickly and with confidence over large areas. 
The geologist unassisted by aerial photographs is forced to rely on finding 
outcrops for his geological data. These outcrops may be small, sparsely distrib 
uted, and indeed difficult to find without the aid of aerial photographs. Away 
from the outcrops, the geologist has little information from which to deduce 
the underlying rock, and none at all on which he can map the strike of the 
relic bedding. 
Aerial photographs sometimes give evidence as to the origin of certain rocks 
when the field evidence is equivocal, non-existent, or difficult to obtain. In 
particular, it has been observed that they sometimes indicate: 1. whether 
certain quartz schists in the Basement Complex originated from the meta 
morphism of siliceous sediments or quartzitic intrusions; and 2. whether cer 
tain granitic rocks were intruded or resulted from the granitization of meta 
sediments in situ. 
Where aerial photographs indicate that a granitic body has moved into 
place, they have already done much towards indicating the origin of the body 
(fig. 6). Also within areas of intruded rocks, aerial photographs may be useful 
instruments of research in that they may reveal genetically significant sub 
divisions of apparently simple intrusions (fig. 6). 
Conversely, when the aerial photographs show no disturbance or pushing 
apart of the country rock by the granitic body, the photogeologist can begin 
asking himself whether the granitic body can have resulted from the processes 
of granitization. If, in this latter case, he finds lineaments within the granitic 
body consistently parallel or colinear with those of the adjacent country rock, 
he is justified in mapping the rock as a probable autochthonous granitic rock. 
It is sometimes thought that, in all areas, the field geologist can eventually