The study of shear (S-) wave velocities in the Lewisian metamorphic complex of Sutherland, NW Scotland, shows their potential value in modelling lithological variation and porosity. The interpretation of a 40-km refraction line across a major lineament separating different rock units produced distinctive lateral and vertical velocity variations due to compositional change and closing of cracks respectively. Analysis of the vertical velocity variation shows water saturation of cracks, except in the top 400 m where cracks may he partially saturated. The lateral velocity variations, which show very good correlation with lithology, have been modelled using effective velocities of essential minerals and by regression analysis of longitudinal (P-) and S- wave velocity against composition. Adding S-wave data to P-wave data improves the modelling capability here only modestly, but is potentially more useful in identifying separately the proportions of mafic (v. felsic) and hydrated (v. unhydrated) minerals.