Beneath the Carboniferous strata in the Marathon uplift are rocks of Cambrian, Ordovician, and probably Devonian ages, whose total exposed thickness is 2,500 feet. Of these, the most completely represented is the Ordovician, which includes representatives of the Beekmantown, Black River (?), Trenton, and Cincinnatian of the standard sections, with a possible partial representation of the Chazy. The system is divided into five formations, four of which are new. The Cambrian is represented by sandy strata of which only the highest beds are exposed, and the Devonian is probably represented by cherts and novaculites. The pre-Carboniferous rocks of the district are of geosynclinal facies, with terrigenous clastic members. They have little in common, in either lithology or faun s, with the sections near by, at Van Horn, Big Lake, or the Llano-Burnet uplift. They have many resemblances to the section in the farther removed Ouachita Mountains, where the strata are also prevailingly clastic, with many siliceous beds, and with graptolites as the chief element in their faunas. This is thought to be the result of the influence of the old land mass of Llanoris.