152 
The basic intrusives are mainly of gabbroic compo 
sition, forming rounded to elongated bodies. They 
form high relief areas and appear as very dark-toned 
areas on the MSS and LFC black-and-white images. 
The related pediment slopes are also dark in tone in 
contrast to most other rock types which have very 
light-toned pediments and fan deposits. On the radar 
images the relief and the relatively high backscat- 
ter together with the circular pattern are indicati 
ve for this rock type. 
The younger granites which are more calc-alkalic 
in composition are typical for high level complexes 
(Vail, 1979). They often form ring-complexes in 
which an extrusive phase may also be present in the 
form of rhyolite or ignimbrite flows. The central 
igneous complexes are round in outcrop pattern and 
also form subdued relief, often sand covered, al 
though the ring-dikes and cone-sheets forming the 
outer ring often form a resistent rim of positive 
relief. Extensive dike swarms are related to these 
igneous complexes and similar to the ring-dikes 
forming resistent outcrops in the aeolian sand 
sheet. It are these dike swarms which have our 
special attention as they may have become covered by 
a thin wind-blown sand cover (fig. 3). 
Figure 3. Dikes outcropping in sand-covered area of 
older granitoid (aerial photograph - eastern desert, 
Egypt. 
Locally, some younger Mesozoic to Tertiary acid to 
intermediate intrusives and extrusives occur as 
stocks and plugs and small lava flows. Also younger 
dikes and veins are known to occur. Some unconform- 
able Nubian sandstone remnants are found in the 
southern extremity of the studied radar strip. 
OCCURRENCE AND ORIENTATION OF DIKE SWARMS IN SANDY 
AREAS 
For a comparative study of the occurrence of dike 
swarms in sandy areas on radar image and on LFC 
photographs, a number of small areas were selected 
on a radar strip (SIR-B data take 97.5) running in 
direction 142° to true north. 
High contrast prints were made of the LFC film on 
Ilford Multigrade 44M paper to enhance tonal differ 
ences in the selected sandy areas. Small rock out 
crops and dikes are better interpretable in the sand 
sheets; the rocky areas, on the other hand, become 
entirely oversaturated (fig. 7). Therefore a normal 
LFC print was used next to the high contrast one. 
From northwest to southeast, the following areas 
were selected. 
Area 1: South of Abu Swayel (Egypt 22°52'N, 
34°38'E). This area of 16 by 17 km is situated in 
the batholitic granitoid, which crops out only in 
the northeast corner of the area but is principally 
sand-covered over the rest of the area (fig. 2). A 
number of discrete dikes crop out continuously or 
as discontinuous patches in the sandy plain. 
The total length of the dikes measured from the 
radar is 125.1 km against 150.1 km from LFC photo 
graphs. Most of the dikes not interpreted on radar 
but clearly visible on the LFC photos occur in the 
rocky higher relief areas. They run partly parallel 
to the radar look direction (fig. 4). 
Lineaments (dikes) interpreted from LFC 
Lineaments from Radar, not present on LFC 
Figure 4. Area 1: South of Abu Swayel (Egypt). 
Dike lineaments interpreted from LFC and SIR-B 
images. 
Some very faint lineaments which appear on the 
radar image in the sandy areas were not conclusive 
ly interpreted from the LFC photo. The possibility 
that we were dealing here with microwave penetra 
tion in the sand cover and backscatter response 
from a burried dike is not unlikely but could not 
be proved convincingly without fieldwork. 
Area 2: Mishbeh (Egypt 22°43'N, 34°41'E). The 
Mishbeh area is 30 by 25 km. Dike swarms run 
slightly radially outwards from the central nephe- 
line syenite plug, but principally in two main 
directions: NNW-SSE and ENE-WSW (fig. 5). 
With exception of one, no dikes are seen cross 
cutting or penetrating into the syenite body, so 
they are considered as penecontemporaneous or 
slightly younger. The age of the nepheline syenite 
is 150 Ma on the basis of Rb/Sr isotopic dating 
(Hashad, 1980). 
A total length of 314.2 km of dikes were inter 
preted from the radar image against 336.1 km from 
LFC photos. The distribution of directional classes 
is clearly bimodal on both rose diagrams (fig. 6). 
It is particularly the ENE-WSW direction which is 
underrepresented on the radar image compared with 
the LFC photo. This direction coincides with the 
radar look directions, which is 52° from true 
north. 
On the LFC photo, the NNW-SSE direction is underre 
presented or overrepresented on the radar, as it is 
the direction oriented perpendicular to the radar 
look direction. Close visual scrutiny of dike lin 
eaments present on radar but not on the LFC photos 
did not indicate that penetration through super- 
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