The lithology and primary structural features of the Ordovician rocks of Victoria, and also of many of the Silurian rocks, are explicable according to the hypothesis of turbidity currents, but it is necessary to account for the preservation of a very complete succession of graptolite-bearing horizons in these rocks. It is suggested that the graptolites which are fossilized within graded beds, occurring towards the top in their silty or clayey portions, represent communities killed by fine suspended detritus in the upper parts of turbidity currents. Each graded bed is thus taken to represent a very small portion of time, and the duration of the diastems on major bedding planes is, correspondingly, much greater. A review of sedimentation and vulcanicity within the Victorian geosynclinal zone in the lower Palaeozoic is given.