WORKING GROUP 2 
GREENWOOD 
99 
elevated bounding wall of schistose metasediment (fig. 3). Dioritic rocks form a 
similar upstanding boundary to granite near Khawrah in Aden Protectorate 
(fig. 4). Although the composition of granite renders it more resistant to 
chemical attack than more basic plutonic rocks and its coherence is greater 
than many pelitic schists, it is here preferentially reduced through its high 
response to thermal disintegration. 
In some cases the relief features of bodies of granite, and diorite or gabbro 
may indicate a comparable degree of reduction. The lesser resistance to chem 
ical weathering of the basic rocks is balanced by the reduced resistance of 
granite to thermal effects. In the basic rocks susceptibility to chemical decay 
results in a more integrated and more deeply incised drainage system and 
watercourses are wider. 
On aerial photographs of Aden basement rocks, granitised metasediment 
and autochthonous granite dominate adjacent intrusive granite, due partly to 
superior jointing in the latter and to foliation and included country rock in 
the former. 
The superior resistance of andesitic volcanic rock vis-a-vis granite is well 
displayed on photographs of the Burum area in the Aden Protectorate. Vol 
canic peaks stand above the reduced granite. Here thermal weathering, to 
which andesite is less responsive, appears dominant. 
Dyke rocks in the Aden basement very generally form positive features in 
their country rock. The finer of the hypabyssal rock is inimical to thermal 
disintegration.