  Consider an $F$-crystal over a noetherian scheme $S$. De Jong--Oort's purity
theorem states that the associated Newton polygons over all points of $S$ are
constant if this is true outside a subset of codimension bigger than 1. In this
paper we show an improvement of the theorem, which says that the Newton
polygons over all points of $S$ have a common break point if this is true
outside a subset of codimension bigger than 1.
