New techniques of X-ray fluorescence spectrography have extended the lower atomic-number limit of determination to 0.01 weight per cent Na in silicates. As a result, chemical analyses of coarse-grained rocks can now be obtained more easily than modal analyses. This paper is a discussion of results obtained from nearly 1000 new rock analyses of samples from the Rattlesnake Mountain pluton, San Bernardino Mountains, California.
The new method of obtaining chemical data from granitic rocks has necessitated reexamination of sampling methods and designs. Samples must be large enough to be rocks, not merely fragments of constituent minerals. A portable diamond drill is a practical means of decreasing bias and extending the population sampled.
Trend-surface analysis of individual elements is a useful computational technique for establishing general patterns of distribution. Factor (vector) analysis makes it possible to study simultaneously the variation of many elements. A more conventional way of achieving this result is to recast chemical analyses as norms; nearly 800 such norms have been computed for this paper. High-speed computers and automatic plotting devices have proved essential for the processing and reduction of our chemical data.
Hierarchical sampling enables one to analyze the variance and to measure its components on different scales, information which is essential to the justification of supposed chemical differences between rocks. Chemical mapping requires knowledge of the variance within localities, and more than one sample must be collected at each locality if this variance is to be measured. It is concluded that the analytical precision is more than adequate for this geochemical purpose; local variability in the rocks themselves is usually the limiting factor.