  We give a direct geometric interpretation of the path model using galleries
in the $1-$skeleton of the Bruhat-Tits building associated to a semi-simple
algebraic group. This interpretation allows us to compute the coefficients of
the expansion of the Hall-Littlewood polynomials in the monomial basis. The
formula we obtain is a "geometric compression" of the one proved by Schwer, its
specialization to the case ${\tt A}_n$ turns out to be equivalent to
Macdonald's formula.
