This great discovery of the order of succession of the British stratified rocks, established by their fossil contents, is due to William Smith, the father of English geology. After exploring the whole of England, he published in 1815 a geological map, the result of his extraordinary labours. Before then people had no idea of a definite and regular succession of rocks extending over the country, capable of being recognised to some extent by the nature of the rocks themselves,--whether sandstones, clays, or limestones, etc., but chiefly by their own fossils. They thought the different kinds of rocks were scattered promiscuously up and down the face of the country; but _now_ we know that they do not show themselves in this haphazard way, but have definite relations to each other, like the many volumes of one large book. By combining the two principles referred to above, geologists have arranged the great series of British stratified rocks into certain groups, each indicating a long period of time. First, they are roughly divided into three large groups, marking the three great eras into which geological time is divided. Secondly, these eras are further divided into certain periods. These periods are again divided into epochs, indicated by local divisions of their rocks. In this way we have something like a historical table. Omitting the small epochs of time, this table is as follows, in descending order:-- _Table of the British Stratified Rocks._ ERA. PERIOD. PREVAILING TYPE. { Recent. Cainozoic, { Pleistocene, or { or Tertiary. { Quaternary. Mammals. { Pliocene. { Miocene. { Eocene. { Cretaceous. Mesozoic, { Neocomian. or { Jurassic. Reptiles. Secondary. { Triassic. { Permian. { Carboniferous. Fishes. { Devonian, and Palæozoic, { Old Red Sandstone. or { Silurian. Creatures without Primary. { Cambrian. a backbone { Archæan,[34] (invertebrates). { or { Pre-Cambrian. [34] The Archæan rocks are frequently placed in a separate group below the Palæozoic.