The test of age by rock-structure has a more restricted use, but is of value when taken in conjunction with the sequence of the strata and the character of their included fossils. To explain both the valuable and the uncertain elements of this last method as a determinant of age, we may cite, for instance, the Upper Ordovician slates of Victoria and New South Wales as an example of uniform rock formation; whilst the yellow mudstones and the grey limestones of the Upper Silurian (Yeringian series) of the same states, are instances of diverse lithological structures in strata of similar age. A reference in the latter case to the assemblages of fossils found therein, speedily settles the question. (3). Hence, the structure and composition of the rocks (lithology), gives only partial evidence in regard to age. =Strata Vertically Arranged.--= The Stratigraphical Series of fossiliferous sediments comprises bedded rocks from all parts of the world, which geologists arrange in a vertical column according to age. A general computation of such a column for the fossiliferous rocks of Europe gives a thickness of about 14 miles. This is equivalent to a mass of strata lying edgewise from Melbourne to Ringwood. The Australian sediments form a much thicker pile of rocks, for they can hardly fall short of 37 miles, or nearly the distance from Melbourne to Healesville. This vertical column of strata was formed during three great eras of time. The oldest is called the Primary or Palaeozoic ("ancient life"), in which the animals and plants are of primitive types. This is followed by the Secondary or Mesozoic ("middle life"), in which the animals and plants are intermediate in character between the Palaeozoic and the later, Cainozoic. The third era is the Tertiary or Cainozoic ("recent life"), in which the animals and plants are most nearly allied to living forms. These great periods are further subdivided into epochs, as the Silurian epoch; and these again into stages, as the Yeringian stage. Vertical Column of Fossiliferous Strata, Australia.