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SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
Fig. 4. 
Differential weathering in arid areas 
Intrusive granite (Gfi preferentially reduced in schists (X) and partially granitised rock (GX). 
Note resistant autochthonous granite (G) 
Beihan area, Aden Protectorate 
R.A.F. Photograph. Crown Copyright Reserved 
the limestone is the dominant rock, and in contrast with the latter, quartzite 
often displays a less rugged and more rounded form. Although under arid 
conditions chemical reduction of limestone - by solution - is minimal, quartzite 
is yet more chemically resistant. The topographical dominance of the lime 
stone is due to its greater resistance to thermal disintegration. The extreme 
resistance to total weathering of both these rock types is a valuable guide to 
identification. 
Clearly demonstrated on aerial photographs is the high degree of reduction 
suffered by intrusive granite. In numerous instances in Aden and Somaliland 
granite areas are marked by plain-like arenas studded with inselbergs and 
exfoliated kopjes. The Las Bar granite of Somaliland is surrounded by an