Two igneous intrusions (sills) emplaced in black shale (Cambrian).
The sills represent the first phase of volcanic activity, but the magma likely never made it to the surface.
This phase was likely part of the first phase around 300 Ma. The first lava flows developed in Brunlandnes and Skien, then prograded north to Jeløya and the Oslo area.
The sills: A range of compositions
Camptonite (mafic, amphibole-bearing)
Rhyolite (yes, actually)
The lower sill at Kontraskjæret: 51 wt% SiO2 (basalt)
The upper sill at Kontraskjæret: 69 wt% SiO2 (rhyolite)