Geological and geophysical observations have defined a 2-layered crust, composed of a 'granitic' layer and an underlying 'basaltic' layer. Some geologists have evoked a pristine granitic layer that intrudes higher level supracrustal rocks by subsequent refusion. In the southern Norwegian Precambrian, the Bamble rocks, whose origin is unquestionably supracrustal, are in fault contact with the Telemark gneisses, deeper level augen gneisses, migmatites, and banded gneisses of granitic composition. Although rocks whose composition is indicative of a supracrustal origin are rare in the Telemark gneisses and their composition commonly approximates the minimum-melting composition in the granite system, the layering universally present in these rocks probably reflects a tectonized depositional fea ture. A number of granitic plutons are emplaced as diapirs, domes, and axial culminations within the Bamble rocks and their equivalents; however, known granitic plutons are rare within the Telemark gneisses. The gravimetrically interpreted thicknesses of these plutons range from 1 to 4 km. Small positive gravity anomalies in the Telemark gneisses further demonstrate the heterogeneity of these deeper level rocks. The model proposed for the granitic crust is this: Supracrustal rocks of intermediate composition underlain by heterogeneous gneisses of more granitic composition. The granitic gneisses were mobilized and rose into the overlying supracrustal rocks to form the granitic plutons. No evidence of a pristine granitic crust is found; the granitic plutons may very well be underlain by gneisses in depth. In order to emphasize the heterogeneous nature of the upper crust, the term, migmatitic layer is more appropriate than 'granitic' layer.
The nature of the "granitic" layer of the crust in the southern Norwegian Precambrian.