Geological time, like all other kinds of time, can only be measured by the succession of events. If the events considered are constant in their recurrence and uniform in their nature, their number will afford the means of measuring the length of time. Thus the oscillations of a given pendulum, the rotation of the earth on its axis, and its revolution round the sun are sufficiently constant and uniform to afford a basis for our seconds, days, and years. Of such uniform and constantly recurring events geology affords no examples by which we could measure the length of geological time.