K-Felspars of the Roffna-porphyries belong to the group of "triclinic orthoclase". Their optical properties (indicatrix orientation) are similar to those of anorthoclase or of a felspar intermediate between anorthoclase and microcline. Homogenous crystals show 2V values ranging between —76° and —86°. Spotted crystals have a greater range of variation, from — 42° to — 84,5°. Concerning the genesis of the "porphyries", the author suggests that they are of detritic origin, resulting from the erosion of albite-rich rhyolite (quartz-porphyries).