In the Sarmiento area occur numerous sills, ring dykes and radial dykes of alkaline basic rocks, principally teschenites, with differentiates of nepheline, analcime and zeolite syenites. They intrude tuffs of the Bajo Barreal Formation. Analcime basanite lavas are widespread in the area. The teschenites consist of plagioclase (An58-20), ophitic titanaugite, some larger crystals having thin aegirine-augite rims, olivine, interstitial analcime, a little zeolite, magnetite, apatite and rare biotite and nepheline. The syenitic differentiates, which are found only in some of the sills, occur as veins up to 40 m thick and may be pegmatitic. They comprise alkali feldspar, rare plagioclase, nepheline, interstitial analcime, sometimes zeolite, pyroxene zoned from augite to aegirine, rare arfvedsonite and olivine, opaques and apatite. Analyses are available in Pezzutti and Villar (1978).