Scattered records of wells drilled to the pre-Cambrian have been gathered from publications, individuals, and the files of oil companies and State and Federal agencies. The revised and consolidated list of wells (Table II) shows as accurate a compilation as possible with corrections, adjustment of differences, and critical evaluation. Many of the data have been taken from insoluble-residue studies or samples. Many deep wells not reaching the pre-Cambrian are included because insoluble residues made it possible to subdivide the Arbuckle group, and, with regional consistency in thickness of the subdivisions, the depth to the pre-Cambrian was computed in many places. The insoluble residue data from individual wells made it possible to compute the amount of Arbuckle eroded, a d regional data based on insoluble residues allowed estimation of the amount of erosion. Consolidation of the data made it possible to either compute or estimate the original thickness of pre-Simpson beds.

A map showing the original thickness of the pre-Simpson beds indicates the topography of the pre-Cambrian surface (Fig. 1). This map gives a picture entirely different from a map drawn on the present elevation of the top of the pre-Cambrian, because the surface is rotated upward to a datum from depths up to 3,700 feet subsea elevation.

A large, rugged, very obvious highland with relief of 1,800 feet is called Tulsa Mountains. It slopes in all directions and has several major extensions. There is a lowland in southern Kansas, and three local highlands are in the extreme northeast corner of Oklahoma, one of which involves a pre-Cambrian fault at Miami.