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.
