This paper is a short description of the insoluble residues obtained from the Cambro-Ordovician magnesian limestones of the Lehigh Valley, Pennsylvania. The methods of collecting the samples and preparing the residues are briefly described. The residues thus obtained are composed of shale, sand, dolocasts, and chert, with tests of arenaceous foraminifera at certain horizons, and almost no heavy minerals. By plotting the proportions of the various constituents in the residues it was found possible to establish certain definite stratigraphic horizons in the series. The foraminifera were especially useful in this work and the zones thus established were used to work out the structure and stratigraphy of the area. A table is given showing the format ions differentiated by this method.