The practicability of estimating chemical variation in a rock from the mode is investigated. Using the modal percentages, determined with a Dollar integrating micrometer, and standard chemical compositions of minerals, the molecular proportions were calculated and von Wolff values determined and plotted. It was found that a good idea of the chemical variation could be obtained and that the accuracy of the modes and derived chemical analyses is consistent within a suite of rocks. The method was used in examining the contact relations of a granite and quartzite in northwest Donegal, Ireland, and the variability of the granite.