Petrographic features of samples taken along a profile from the contact towards the centre of a dolerite dyke are described. The incidence of twinning in plagioclase is high near the contacts and decreases progressively towards the centre of the dyke. The ccnverse is observed with respect to pyroxene twinning. It is suggested that the twinning in pyroxene is secondary and represented by two genetic types: glide twinning and synneusis twinning.
On pyroxene twinning