stripes
El Naga defines a horseshoe-shaped ridge some 4 km in diameter completely surrounded by wadi alluvium so that no outer contacts are exposed. However, on the outer side of the wadi in the northeast and southeast, beyond the area shown on Fig. ??, Precambrian gneisses are slightly fenitized with reddening of feldspars and development of amphibole. The outer part of the complex consists of coarse rocks described as umptekite consisting of perthite, a little nepheline and analcime, alkali pyroxene and amphibole, biotite and accessories. The central area is dominantly of nepheline syenite with 30% or more of nepheline, which may be replaced by zeolites and aegirine-augite. There are remnants of a volcanic cone in the central area which include trachybasalt, trachyte, phonolite and a range of pyroclastic rocks. They are cut by apophyses of the umptekite and nepheline syenite which have caused much alteration and the production of hybrid rocks. Numerous radial and ring-dykes up to 2 m thick consist of a range of microsyenites and phonolites. Modal and chemical data are given by El Ramly et al. (1969b and 1971) and trace element data by Abdel-Aziz and Hassan (1973) and Soliman (1987). M.S. Garson, in an unpublished report, refers to radially disposed, coarse-grained calcite veinlets on the north-eastern part of the central stock which contain up to 300 p.p.m. Nb. He also notes a record of a radial carbonatite dyke to the south-east of El Naga.
