A. K. TURNER Department of Geology and Geological Engineering Colorado School of Mines Golden, Colorado 80401 USA Geology deals with three-dimensional data. Geoscientists are concerned with three dimensional spatial observations, measurements, and explanations of a great variety of phenomena. The representation of three-dimensional data has always been a problem. Prior to computers, graphical displays involved specialized maps, cross-sections, fence diagrams, and geometrical constructions such as stereonets. All were designed to portray three-dimensional relationships on two-dimensional paper...