Peridotite and gabbro form much of the eastern peninsula (Banggai).
Lithologically they are crystalline schists, together with granite, diorite, gabbro and other igneous rocks.
Geologically, its northern half is composed of Torridonian sandstone, with basalt at points between the West coast and the centre, of gabbro in the south-east, with a belt of gneissose rocks on its east seaboard and of quartz-porphyry in the south-west.
A remarkable feature in the volcanic phenomena was the disruption of the basaltic plateaus by large bosses of gabbro and of various granitoid rocks.
Intrusions of hyperite, gabbro (anorthite-gabbro at Radmanso in the province of Stockholm) and diorite are also abundant.
The Spiti shales are succeeded conformably by Cretaceous beds (Gieumal sandstone below and Chikkim limestone above), and these are followed without a break by Nummulitic beds of Eocene age, much disturbed and altered by intrusions of gabbro and syenite.
The oldest rocks exposed are gneiss, talc-schist and serpentine, with intrusive masses of gabbro and diabase.