Among the basaltic rocks of the Rio Chubut Medio are rocks identified as pantellerites.
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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.