The division of geological history into larger and smaller periods does not imply the recurrence of sudden revolutions; it is in some measure dictated by considerations of convenience, but more particularly by our ignorance of certain stages in the history of the world due to the imperfection of the record. GEOLOGICAL TABLE. Showing the position in the Geological Series of the strata referred to in this volume. Tertiary (Cainozoic) { =Recent= { Superficial accumulations containing human { =Pleistocene= { remains (Metal age, Neolithic and { { Palaeolithic ages, Glacial deposits) { { =Pliocene= Cromer Forest-bed, etc. { { =Miocene= Absent from Britain. { { =Oligocene= Bovey Tracey beds, etc. { { =Eocene= London Clay, etc. Secondary (Mesozoic) { =Cretaceous= { Chalk { { Wealden beds { { { Purbeck and Portland beds (Upper Jurassic) { =Jurassic= { Oolites (Middle Jurassic) { { Lias (Lower Jurassic) { { { Rhaetic beds { =Triassic= { Keuper " (Marls with rock-salt, etc.) { { Bunter " Primary (Palaeozoic) { =Permian= { Red Sandstones, etc. { { Magnesian limestone { { { Coal Measures { =Carboniferous= { Millstone Grit { { Carboniferous limestone { { =Devonian= { Devonian limestones, etc. { { Old Red Sandstones { { =Silurian= { Sandstones, shales, { { some limestone { { =Ordovician= { Slates, sandstones, { { Volcanic rocks, etc. { { =Cambrian= { Slates, Sandstones, etc. { { =Pre-Cambrian= { Slates, Volcanic rocks, etc. { =or Archean=