This is a basalt, a volcanic rock formed from a magma of
basic composition erupted on the Earth's surface. It is dark
coloured and fine grained - crystals cannot be distinguished
in it. The large green lumps, however, are xenoliths,
literally "foreign rocks", plucked from the source region of
the magma or from the walls of the cracks the magma passed
through. In this case, the xenoliths are peridotites from
the mantle itself, where the basalt magma was generated.
They are rich in green olivine.