The slip systems of orthorhombic olivine can be advantageously described in the almost perfect hexagonal close-packed lattice of the oxygen ions whose basal plane corresponds to (100). Possible dissociations of dislocations in prism planes of the oxygen lattice are given in a hard sphere model. It is proposed that thermally activated recombination of <100> screw dislocations may lead to cross-slip on portions of prism planes. Such composite cross-slip could account for the noncrystallographic {Ok1}<100> slip referred to as 'pencil glide' in the current literature. The dissociation of dislocations appears to be accompanied by a small increase in volume. It is suggested that hydrostatic pressure may favor the recombination of dislocations, hence cross-slip.