The present paper contains optical data obtained on various feldsparsduring heat treatment.
The experiments were carried out by heating ordinary thin sections in a nichrome furnace. The changes in the optical properties caused by the heat were observed with a petrographic microscope provided with a Federow stage.
The writer is indebted to Dr. Olaf Andersen of the ResearchLaboratory of the U. S. Steel Corporation, Kearny, New jersey, fora specimen of the single microcline from Evje (see experiment No. 9).