Pieces of Darwin Glass, a frothy siliceous glass occur in soil overlying sandstones, limestones and volcanic rocks of Cambrian to Silurian age in Western Tasmania. The refractive index averages 1.480. No radiogenic argon was detected, indicating a maximum possible age of 10 6 years. The glass is not uniform in composition, and at least two distinct, but closely related groups exist. The average chemical composition is as follows: Si, 40.0% (85.62% SiO 2); Al, 3.52% (6.64% Al 2O 3); Ga, 16ppm; Cr, 115 ppm; Fe 3+, 1.63% (2.34% Fe 2O 3); Mg, 0.58% (0.96% MgO); Li, 3.7 ppm; Ti, 3520 ppm (0.59% TiO 2); Ni, 122 ppm; Co, <3 ppm (24 ppm in Group I); Cu, 10 ppm; V, 29 ppm; Fe 2+, 1.29% (1.66% FeO); Total Fe, 2.57%; Zr, 390ppm; Mn, 135 ppm; Sc, 4.1 ppm; Y, 30ppm; Na, 370ppm (0.051% Na 2O); Ca, 740 ppm (0.10% CaO); Sr, 14 ppm; Pb, 7.6 ppm; K, 1.50% (1.81% K 2O); Ba, 340ppm; Rb, 78 ppm; Cs, 3.6 ppm. The analytical methods are given, with values for the precision, and the standard samples used for calibration. A detailed geochemical comparison is made with terrestrial rocks, impact glasses, australites, and with the adjacent country rocks. Darwin Glass is not formed by terrestrial igneous processes. The chemical composition does not resemble that of the underlying rocks, but bears a close resemblance (except for nickel) to that of an argillaceous sandstone. The relative abundances of the alkali elements indicate that little selective loss of the cations considered, has occurred during melting, and the present composition is close to that of the parent material. Darwin Glass is not related to the australites. It closely resembles terrestrial impact glasses, particularly in the concentration of nickel and the Fe/Ni ratios.