The situation in which the speed at which a seismic wave travels varies according to the direction of travel. While most minerals are anisotropic (see anisotropy) the mineral grains forming a rock are randomly aligned, so the anisotropies usually cancel out, leaving the rock isotropic at the wavelengths of seismic waves (metres to hundreds of metres). Foliation tends to align mineral grains, however, so large masses of foliated rocks are often anisotropic. Rocks such as shale are also seismically anisotropic because they are composed of elongate grains that align during the deposition that eventually forms the rock.