The relations in ores between opaque and translucent minerals can not be determined satisfactorily by independent examination of either polished surfaces or thin sections. Previous attempts to prepare polished thin sections by Tolman and Rogers, and thinned polished sections by Donnay, appear not to have been entirely successful. Canada balsam, the cementing medium used, is not sufficiently tenacious to overcome the stresses imparted to the surface of the polished grain by the polishing process, which cause the grains to crackle and curl away from the glass slide. This paper describes two mounting media which largely overcome these stresses. These media are the thermoplastics glycol phthalate and Gelva V 7. Glycol phthalate is best suited to the polishing of thin sections on a cloth lap. Gelva V 7 is preferable if sections are to be polished on a Graton-Vanderwilt polishing machine. Photomicrographs of the results are appended.

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