The remarkable swarm of inclined basic intrusive sheets found in the deeply dissected part of SE Iceland is believed to constitute part of a widespread layer which, in other parts of the country, occurs below sea level as the 6.35 km s−1 crustal layer revealed by seismic refraction studies. A simple mechanism for the generation of the sheet swarm is proposed, based on the contrasted density gradients of crust and uprising magma: basic magma rises to the surface when its density is everywhere less than the bulk density of the rocks it cuts, otherwise it is often diverted laterally to form an intrusive sheet where its density equals that of the country rock. Once initiated, density relations in the crust are such that the swarm is strongly self-perpetuating in nature. It is believed that more than half of the uprising magma in Iceland has been so diverted to form the sheet swarm. One corollary is that the crust in Iceland “filters” the magmas entering it so that only the lighter or those which rise at a high volumetric rate succeed in passing through to the surface. Confluent sheet gabbro intrusions may develop when the frequency of uprise of magma batches is high. Sheet swarm cupolas or perched swarms also occur in central volcanoes, in which the low-density acid volcanic rocks “capture” uprising magmas.