Nepheline basalts form the plugs of La Leoncita, La Madera, La Piedra and La Garrapata, all having diameters of about 1 km, while the plug of Cerro Piedra is much smaller. They cut Precambrian micaceous gneisses and form prominent hills. The rock of the plugs comprises olivine, pyroxene, nepheline, perovskite and magnetite and in some varieties biotite and analcime. Cortellezi et al. (1981) report 16% modal leucite in a rock from a quarry in La Garrapata, the pyroxene of this rock being aegirine-augite.