Soft clay-bearing and otherwise friable rocks that cannot be shaped or mounted without preliminary treatment are impregnated with lucite monomer. Polymerization of this agent is accomplished overnight by accelerated heating and use of a sand filler around the specimen. After this impregnation even montmorillonite-rich samples need be shown no additional special attention throughout the remainder of the routine of sectioning.All sections are mounted with Lakeside No. 70 thermoplastic cement. Then, instead of grinding away the excess rock with coarse abrasives, thin sections are cut rapidly and evenly to about 0.08 mm. or less on a specially built diamond saw. Finishing down to 0.03 mm. may then be accomplished by hand on a power lap or glass plate, for covering in the usual manner, or the sawed sections may be placed in adjustable holders on a polishing machine for mechanical finishing and polishing, as desired.