An investigation of the subsurface Lower Ordovician and Upper Cambrian limestones and dolomites of north-central Texas was made mainly by a microscopic examination of well cuttings and siliceous residues derived from the cuttings, during the fall of 1943 and the spring of 1944. These rocks are generally known as the Ellenburger limestone. They were subdivided into six units of which two were further subdivided into two sub-units each. Each unit and sub-unit contains siliceous residues that characterize it and that persist throughout the region. Eight cross sections show the correlations of these subdivisions from well to well. Unit A, the youngest of the subdivisions, is present only in the eastern and northeastern parts of the region where it has been preserved in a regi nal syncline. The distribution of the other units is erratic because at least one period of erosion during, and several such periods following, the deposition of these beds have removed unequal parts of them throughout the region.