- GeoRef, Copyright 2008, American Geological Institute. Reference includes data from Bibliography and Index of North American Geology, U. S. Geological Survey, Reston, VA, United States
The method involves study of a series of sedimentary rock samples with known values of median diameters and carbonate percentages, and formulation of a "mental image" of the values. These samples then serve as references, on the basis of which unknowns are estimated and checked by laboratory techniques. Estimates are revised accordingly.