Primary and secondary fell out of use altogether, while quaternary was discarded by some but kept by others. Today only tertiary remains as a common designation everywhere, even though it no longer represents a third period of anything. Lyell, in his Principles, introduced additional units known as epochs or series to cover the period since the age of the dinosaurs, among them Pleistocene (“most recent”), Pliocene (“more recent”), Miocene (“moderately recent”), and the rather endearingly vague Oligocene (“but a little recent”).
