In the Sudan, the principal difference between the Arabian-Nubian shield and the foreland to the west is the uniformly juvenile isotopic signature of the former and the generally higher metamorphic grade of the latter. There generally is not a significant difference in radiometric ages between the two basement tracts, but a boundary can be drawn between ensimatic crust in the east and ensialic crust in the west, based on the aforementioned isotopic and metamorphic discriminants. Differences in metamorphism and deformation and the identification of a conglomerate indicate that there are two distinct Precambrian successions. The older one contains strongly foliated and isoclinally folded rocks which in the field appear to be amphibolite-facies gneisses but which under the microscope often contain a greenschist-facies assemblage. These gneisses are exposed along the east side of the Nile near the Duweishat gold mine. The younger Precambrian succession is less deformed and metamorphosed and includes predominantly felsic and subordinate mafic volcanic rocks and volcaniclastic sediments.