Large sodium cobaltinitrite-stained slabs of hand specimens are magnified under a binocular microscope and counted on a large mechanical stage. The method is recommended for rocks that are so coarse grained as to require a very large number of thin sections per specimen for a moderately precise estimate of their mode, but not so coarse grained as to make field identification of minerals a reliable procedure.

The method is applied to a quartz-mangeritic augen gneiss, and the variability between the specimens is broken down into components of vaiance: (1) The operator plus counting component of variance is not significantly different from binomial expectation. (2) The within-specimen component of variance is used to predict the number of slabs required per specimen to obtain a mode of given precision. (3) The between-specimen component of variance is unknown, but a rough estimate of its maximum size is possible and is useful for designing sampling plans in similar rocks.

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