Partial trend surfaces based on coarseness index, a modification of Chayes' (1955) identity change number, are computed for a quartz diorite pluton in the Precambrian complex of northeastern Wisconsin. E. J. Lyons (in Bradley and Lyons, 1953), on the basis of standard field and petrographic criteria, has explained the origin of most plutonic rocks in the area in terms of large-scale metasomatism. Later writers have challenged this hypothesis by different interpretations of the same basic data. Similar observations by the present author, as well as trend-surface analysis of mineralogic composition, failed to provide a solution to the controversy. Coarseness-index trend surfaces, however, do require a centrally located control of coarseness within the quartz diorite pluton and indicate that a linear trend component is practically nonexistent for this unit. These findings suggest that the existing metasomatic theory should be abandoned or substantially revised.

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