The lower part of the Bungonia limestone contains Ludlovian (Late Silurian) fossils, but no age-diagnostic fauna have been identified from the upper part of the formation nor from the overlying Tangerang volcanic rocks. The K/Ar age determination reported here from the Tangerang volcanics (399 m.y.) appears to be too low, suggesting radiogenic Ar loss. K/Ar data for the S part of the Marulan batholith yield a mean age of 398 m.y. (Early Devonian). Since the Tangerang volcanics overlie the Bungonia limestone and are intruded by the Marulan batholith they are probably Early Devonian. It is suggested that the Tangerang volcanics are a correlative of the Bindook porphyry. -N.C.N.S.