Summary: Sedimentary analcime in a small basin in Cainozoic basalt at Murrurundi, New South Wales, has probably resulted from the reaction of Na-rich water on colloidal nontronite derived from volcanic ash. Based on cell dimension (13.713 Å) and Δ2θ(1·81) determined by Saha's (1959) method it is a Group C low-silica type of Coombs and Whetten (1967).