History of the Time Scale
 Nicholas Steno devised the basic principles of the geological timescales at the end of the 17th century. Steno was the first person to pay attention to the strata of rock layers containing evidence of the past, and determined what now seems to be the obvious idea that the lower strata are the oldest, and each higher level was created more recently. Steno realized that two similar strata represented the same geological time, even if one widely-separated part was tilted, twisted, buckled or inverted with respect to the other part. Identifying strata by the fossil content wasn't done until the 19th century.
The earth-wide geological time scale was first formulated in the late 18th century by Abraham Werner and other geologists, dividing the earth's crust into Primary, Secondary, Tertiary, and Quaternary types of rocks, with each type relating to a particular period in the history of the earth.
In the early 19th century, Alexandre Brogniart, Georges Cuvier and William Smith pioneered the identification of geological strata by the fossil content, expanding the ideas of Nicholas Steno and allowing earth history to be identified more finely. Since two strata containing the same fossils were likely created at the same time, widely separated strata could be easily linked in time, even if having dissimilar composition.
From 1820 to 1850, the series of geological "periods" were identified b y detailed studies of the geological strata of Europe. The names of the periods reflects the fact that the process was dominated by British geologists. The "Cambrian," "Ordovician," and "Silurian" periods were named for ancient British tribes (and defined using stratigraphic sequences from Wales). The "Devonian" was named for the British county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was delineated and named by British geologist Roderick Murchison, although defined using strata in Russia.
The three-age system
 is a system of classifying human prehistory into three consecutive time periods, named for their respective predominant tool-making technologies: the Stone Age, the Bronze Age, the Iron Age.
Its formal introduction is attributed to the Dane Christian Jürgensen Thomsen in the 1820s in order to classify artifacts in the collection which later became the National Museum of Denmark. Thomsen was not the first to use tool-making materials as a basis for classifying prehistoric societies; the Frenchman, Nicholas Mahudel had proposed a similar system in the early eighteenth century and the idea gathered supporters in the intervening hundred years.
Thomsen and his predecessors argued that nobody would have used stone tools if bronze ones had been available and that similarly, no one would have wanted to use bronze tools if there had been iron ones around instead. Reasoning that the advances must therefore have come in chronological sequence, he suggested this as a workable basis for dating artefacts and sites. Such a system was revolutionary and a vast improvement on the disorganised nature of previous prehistoric archaeology.
Later, the Stone Age was divided into the Palaeolithic and the Neolithic and further subdivisions were introduced to divide all the ages into early, mid or late (or lower, middle and upper in the case of the Palaeolithic) sections. There are also the Mesolithic and Epipaleolithic periods. In some cultures, archaeological evidence has made it necessary to add a Chalcolithic or Copper Age period between the Neolithic and the Bronze Age. The term Megalithic does not refer to a period of time and merely describes the use of large stones by ancient peoples from any period.
The three age system has been difficult to apply fully outside Europe. Cultures developed at different rates or missed out some of the stages of development altogether. Amazonian tribes in South America remain in the Neolithic for example, while there was no Bronze Age south of the Sahara and people living there went straight from using stone to iron.
It also soon became apparent that the switches from one age to another did not happen quickly or decisively. Flint tools remained in use in a limited fashion into the Iron Age in Europe and early metal items often appear in what should technically be the Neolithic.