This draft paper presents field observations which can be used to qualify two Rb/Sr geochronological results obtained from biotites sampled in sheared migmatitic gneiss of the western Yilgarn Block in WA. The site visited lies in a quarry near Northam, where the geochronology samples reported by Libby and de Laeter (1979) were obtained. A sample of biotite from the hornblende gneiss gave an age of 2372 Ma [no uncertainties are given], while a sample from a biotite lens, which forms part of an overprinting shear zone, yielded an age of 2164 Ma. It is proposed that deformation of the earlier biotite-hornblende gneissic foliation within the migmatitic gneiss into narrow shear zones resulted in the resetting of the Rb/Sr system and the commensurate difference in age between the ‘age’ of the main foliation and the ‘age’ of the biotite schist zone.