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

Dolerite (basaltic)

Camptonite (mafic, amphibole-bearing)

Rhyolite (yes, actually)

Mænaites?

The lower sill at Kontraskjæret: 51 wt% SiO2 (basalt)

The upper sill at Kontraskjæret: 69 wt% SiO2 (rhyolite)

Figure from Sundvoll et al., Tectonophysics, 1992. Link here.
Sæther, 1947 – dykes in Bærum
Sills (grey) in Cambrian shale (black) – Gran, Hadeland
Evolution of the Oslo Rift, from Landet blir til.
Sedimentation in the late Carboniferous – and the first basalt in the Oslo area (B1, Kolsåsbasalten). From Landet blir til.
Composition of extrusive rocks (TAS diagram). From Landet blir til.
Rhomb porphyry from the dyke underneath the Akershus fortress.
Outcrops of Ordovician shale and a rhomb porphyry dyke at Tjuvholmen before levelling. Picture from ngu.no.