The method of making the divisions and subdivisions of geological time is not yet a fixed one, and there is much difference in the usage of various writers. The names of the divisions also have been given at various times and in many lands, according to no particular system. Most of these names have been taken from the locality or district where the rocks in question were first studied or are most typically displayed; as Devonian from Devonshire, Jurassic from the Jura Mountains. Some are named from a characteristic or prevalent kind of rock, such as Cretaceous (Latin creta, chalk) and Carboniferous. Of late there has been a tendency toward a more uniform method of nomenclature, and to the use of one set of terms for the divisions of time, and another and corresponding set for the divisions of the strata. The grander divisions of time are called eras, and in descending order we have periods, epochs, and ages. The following table represents the divisions in the scale of time and the scale of rocks which have been adopted by the International Geological Congress.

It will be observed that the subdivision is carried farther in the scale of rocks than in that of time, because of the generally local character of these minor subdivisions. The names employed are, as yet, the same for both scales, and we speak of the Palaeozoic Era or Group, and of the Silurian Period or System. It has been proposed to give separate names to the divisions of the two scales, and this would be an improvement in some respects.

Table Or Major Geological Divisions