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.
| ERA. | EPOCHS IN EUROPE. | EQUIVALENT STRATA IN AUSTRALIA. |
| CAINOZOIC or TERTIARY ([Note 1]). | HOLOCENE | Dunes, Beaches, and Shell-beds now forming. |
| PLEISTOCENE | Raised Beaches, River Terraces, Swamp Deposits with Diprotodon, Cave Breccias, Helix Sandstone. | |
| PLIOCENE | Upper.—Estuarine beds of bores in the Murray basin, Marine beds of Limestone Creek, Glenelg River, Vic. (Werrikooian). | |
| Lower.—Kalimnan red sands (terrestrial) and shell marls (marine) of Victoria, Deep Leads (fluviatile) in part, Upper Aldingan of South Australia. | ||
| MIOCENE | Deep Leads in part: Leaf-beds of Bacchus Marsh, Dalton and Gunning. Janjukian Series of C. Otway, Spring Creek, and Table Cape. Batesford Limestone. Polyzoal Rock of Mt. Gambier and the Nullarbor Plains. Older Cainozoic of Murray basin, Lower Aldingan Series of S. Australia, Corio Bay and Bairnsdale Series. | |
| OLIGOCENE | Shelly clays and leaf-beds of the Balcombian Series at Mornington; also Shell-marls and clays with Brown Coal, Altona Bay, and lower beds at Muddy Creek, W. Vict. | |
| EOCENE | Probably no representatives. | |
| MESOZOIC or SECONDARY | CRETACEOUS | Upper.—Leaf-beds of Croydon, Q. Desert Sandstone,Q. Radiolarian Rock, N. Territory. Gin-gin Chalk, W.A. |
| Lower.—Rolling Downs Formn., Q. Lake Eyre beds, S.A. | ||
| JURASSIC | Marine.—Geraldton, W.A. | |
| Freshwater.—Carbonaceous sandstone of S. Gippsland, the Wannon, C. Otway and Barrabool Hills. Ipswich Series, Q. Mesozoic of Tasmania, Talbragar beds, N.S.W. | ||
| TRIASSIC | Upper leaf-beds at Bald Hill, Bacchus Marsh, Vict. Hawkesbury Series (Parramatta Shales, Hawkesbury Sandstone, Narrabeen beds), N.S.W. Burrum Beds, Q. | |
| PALAEOZOIC or PRIMARY | PERMIAN and CARBONIFEROUS, UPPER | Carbopermian ([Note 2]), Coal Measures of New South Wales, W. Australia, Queensland (Gympie Series) and Tasmania. Gangamopteris beds of Bacchus Marsh, Vict. Upper Carboniferous of Clarence Town, N.S.W. |
| CARBONIFEROUS, LOWER | Fish and Plant beds, Mansfield, Vict. Grampian sandstone; Avon River sandstone, Vict. (?) Star beds, Queensland. Lepidodendron beds of Kimberley, W.A. ([Note 3]). | |
| DEVONIAN | Upper.—Sandstones of Iguana Creek, with plant remains. Lepidodendron beds with Lingula, Nyrang Creek, N.S. Wales. | |
| Middle.—Fossiliferous marbles and mudstones of Buchan, Bindi and Tabberabbera, Vict. Rocks of the Murrumbidgee, N.S. Wales, and of Burdekin, Queensland. | ||
| SILURIAN | Upper.—(Yeringian stage).—Lilydale, Loyola, Thomson River, and Waratah Bay, Vict.; Bowning and Yass (in part), N.S. Wales; Queensland. | |
| Lower (Melbournian stage).—Melbourne, Heathcote, Vict.; Bowning and Yass (in part), N.S. Wales. Gordon R. Limestone. | ||
| ORDOVICIAN, UPPER and LOWER | Slates (graptolitic).—Victoria and New South Wales. (?) Gordon River Limestone, Tas., in part ([Note 4]). Larapintine series of Central Australia. | |
| CAMBRIAN | Mudstones and limestones of Tasmania, South Australia, Victoria and W. Australia. | |
| PRE-CAMBRIAN | Fossiliferous rocks doubtful chiefly represented by schistose and other metamorphic rocks. |