  For an $F^n$-crystal $\mathfrak C$ over a reduced locally Noetherian $\mathbb
F_p$-scheme $S$, Vasiu first obtained the purity of the Newton polygon strata
of $S$ defined by $\mathfrak C$. Deligne later obtained the purity of the
$p$-rank strata of $S$ defined by $\mathfrak C$. We prove a stronger result
that for every fixed point $P_0$ in the $xy$-coordinate plane, the reduced
locally closed subscheme $S_{P_0}=\{s\in S| NP(\mathfrak C_s)\mbox{ has
}P_0\mbox{ as a break point}\}$ of $S$ is pure in $S$ (A weaker result was
known for the weak purity of $S_{p_0}$).
