Pronounced relief of the exposed surface of the Glen Alpine stock offers an opportunity for study of three-dimensional mineralogical variation in a granitic pluton. Polynomial surfaces fitted by least-squares methods to modal data from this stock indicate that, near the center of the pluton, the vertical rate of mineralogical change is large in comparison to the horizontal rate of change; whereas, near the boundaries the opposite is true. Variation of this type is most pronounced in modal potassium feldspar and color index but is present to a minor extent in modal quartz and plagioclase. These results suggest a three-dimensional model of mineralogical variation which may be appropriate for other plutons. Vertical mineralogical gradients are an important feature of this model; consequently, the model implies that the mineralogical composition of each exposed level of a pluton is unique. Thus, one must be cautious in applying observations of a two-dimensional exposed surface to the determination of the genesis of a three-dimensional pluton.

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