  Let $\mathfrak g$ be an affine Lie algebra with index set $I = \{0, 1, 2,
\cdots , n\}$. It is conjectured in \cite{KNO} that for each Dynkin node $k \in
I \setminus \{0\}$ the affine Lie algebra $\mathfrak g$ has a positive
geometric crystal whose ultra-discretization is isomorphic to the limit of a
coherent family of perfect crystals for the Langland dual ${\mathfrak g} ^L$.
In this paper we show that at the spin node $k=6$, the family of perfect
crystals given in \cite{KMN2} form a coherent family and show that its limit
$B^{6,\infty}$ is isomorphic to the ultra-discretization of the positive
geometric crystal we constructed in \cite{MP} for the affine Lie algebra
$D_6^{(1)}$ which proves the conjecture in this case.
