Composite dikes of the Oslo region are characterized by doleritic borders and acid centers, the contact between the associated rocks being sharp in the case of quartzporphyry and transitional in the case of syenite centers. A new example of such a dike is described, the older Iiterature on the subject is critically reviewed and it is shown that all the composite dikes of the Oslo region are built up by successive intrusions.
In the author's mind, composite dikes of subvolcanic origin are normally bound to eruptive provinces in which the magmas associated in the dikes also exist as pure rock bodies within the crust or as lava flows. If a fissure taps differrent local magma reservoirs, the basic magma will flow more readily into the expanding dike than the more viscous acid one. If, however, the expansion continues, the latter will be favored because it will not be cooled in the fissure, which has been heated up by the basic magma, and because of its lower density. Sharp contacts arise, when the associated magmas are of a strongly contrasted composition and have notably different consolidation temperatures. Transitional contacts are produced when these differences are smaller so that mixing occurs in the contact zone. The hybrid nature of the latter is often attested by the simultanuous presence of corroded phenocrysts of both magmas