Plagioclase from different lavas spills, from the southern end of the Paraná Basin, were optically determined by the macla Albita-CarIsbad, Carlsbad and Albita laws, which are the most frequent. In basaltic rocks of uniform granulation, the general composition of plagioclase is between An45-An65, with a concentration of values in the range An50-An55. In unequigranular types, phenocrysts usually show values above An60. In the first case, the plagioclásios are of the volcanic type and of transition; in the latter, phenocrysts are usually of the plutonic or transition type, thus reflecting the thermal history of rocks. The knowledge of the structural state of the plagioclasses of the volcanic rocks of the Paraná Basin favors the precision of the optical measurements, for the calculation of the percentages of anortita,
