Diachroneity is a substantial character of sedimentary strata and its essential cause is sedimentation. Where there is sedimentation there will be diachroneity (lateral accumulation) ; even if diachroneity does not exist in limited areas with vertical accumulation, it must exist in relatively extended scale of such sedimentation. Marine transgression or regression could define the type diachroneity but is not the necessary factor or essential cause for diachroneity.The movement and accretion of plates have caused the diachroneity not only of sedimentary strata but also of volcanogenic strata. This kind of diachroneity has nothing to do with sedimentation and is tectonic diachroneity in this paper.