O conceito de migração implica: O cruzamento de uma fronteira por período mínimo de 6 meses. Envolve todas as deslocações das populações. Diz respeito apenas a movimentações forçadas. Está relacionado com às deslocações de pessoas por mais de 1 ano.

Ageologist is working in an area with exposed sedimentary rocks and finds a well exposed sequence of stratified rocks. as he works through a thick sequence of limestone, he sees abundant large clam fossils that produce an interlocking texture suggesting a reef, but within the limestone section these clams disappear and the overlying limestone is made up primarily of fragments of corals. he now moves 50 km away and finds limestones interbedded with shales, and the limestones contain the same clam fossils he had seen previously 50 km away. as he continues through this section, the rocks become entirely limestone, and the clams disappear again with coral fossils above forming a reef structure. he concludes