layering systems has often been found to be widely inaccurate. Chemical dates therefore are only considered reliable used in conjunction with other methods. They are collected in groups of data points that form a pattern when graphed. Isolated dates are not considered reliable. Other -liths and -lithics The term Megalithic does not refer to a period of time, but merely describes the use of large stones by ancient peoples from any period. An eolith is a stone that might have been formed by natural process but occurs in contexts that suggest modification by early humans or other primates for percussion. Three-age system resumptive table * Formation of states starts during the Early Bronze Age in Egypt and Mesopotamia and during the Late Bronze Age first empires are founded. Criticism The Three-age System has been criticized since at least the 19th century. Every phase of its development has been contested. Some of the arguments that have been presented against it follow. Unsound epochalism In some cases criticism resulted in other, parallel three-age systems, such as the concepts expressed by Lewis Henry Morgan in Ancient Society, based on ethnology. These disagreed with the metallic basis of epochization. The critic generally substituted his own definitions of epochs. Vere Gordon Childe said of the early cultural anthropologists:Last century Herbert Spencer, Lewis H. Morgan and Tylor propounded divergent schemes ... they arranged these in a logical order .... They assumed that the logical order was a temporal one.... The competing systems of Morgan and Tylor remained equally unverified – and incompatible – theories. More recently, many archaeologists have questioned the validity of dividing time into epochs at all. For example, one recent critic, Graham Connah, describes the three-age system as "epochalism" and asserts:So many archaeological writers have used this model for so long that for many readers it has taken on a reality of its own. In spite of the theoretical agonizing of the last half-century, epochalism is still alive and well ... Even in parts of the world where the model is still in common use, it needs to be accepted that, for example, there never was actually such a thing as 'the Bronze Age.' Over-simplification Some view the three-age system as overly simple; that is, it neglects vital detail and forces complex circumstances into a mold they do not fit. Rowlands argues that the division of human societies into epochs based on the