Andesitic basalt of the Hekpoort formation in Faerie Glen Nature Reserve, Pretoria. Percolating water caused accretion of a secondary mineral, so-called amygdales, in the rock vesicles. The vesicles were caused by dissolved magma gasses that effervesced from the lava due to decreased pressure and temperature. The rock is dated to the early Proterozoic, some 2.22 billion years ago.