The rooks composing the earth's crust contain a history and represent time—a history of changes numerous, varied, and important: changes in the distribution of land and water; in the thermal conditions of the world; and in the character of the organic tribes which have successively peopled it. The time required for these mutations must have been vast beyond human comprehension, requiring, for its expression, units of a higher order than years or centuries. In the existing state of our knowledge it is impossible to convert geological into astronomical time: it is at present, and perhaps always will be, beyond our power to determine how many rotations on its axis, or how many revolutions round the sun the earth made between any two recognised and well-marked events in its geological history. Nevertheless it is possible, and eminently convenient, to break up geological time into great periods: it must not be supposed, however, that such periods are necessarily equal in chronological, organic, or lithological value; or separated from one another by broadly marked lines of demarcation; or that either their commencements or terminations in different and widely separated districts were strictly synchronous.

One of the terms in the chronological series of the geologist is known as the Devonian, that which preceeded it the Silurian, and the succeeding one the Carboniferous period; and these, with some others of less importance, belong to the Palæozoic or ancient-life epoch, or group of periods.