The Caballos Novaculite is composed predominantly (90 percent) of bedded chert; the widespread distribution of two novaculite (milky white chert) lithosome marker units permits the formation to be divided into five members. From the base up, the members are listed with maximum thicknesses and rock type as follows: lower chert, 15 feet, light brown and off-white chert with shale partings; lower novaculite, 150 feet; lower chert and shale, 200 feet, chert of many different colors (chiefly green) and shale partings, and several beds of calcarenite; upper novaculite, 400 feet; and upper chert and shale, 400 feet, chert of many different colors and shale partings. Minor amounts of red and green shale, pebble conglomerate, sandstone and calcarenite are in the chert and shale units. The Caballos Novaculite in the Marathon Basin is a lens-shaped unit from 100 to 700 feet thick, based on 26 measured sections. The Caballos is conformable with the underlying Maravillas Formation of Late Ordovician age and with the overlying Tesnus Formation of Mississippian to Early Pennsylvanian age; and, therefore, probably includes rocks of Silurian, Devonian, and possibly Mississippian age. The novaculite members may be time- stratigraphic units in the center of the Marathon Basin. Chert formed by the diagenetic alteration of opaline skeletal particles, chiefly sponge spicules and Radiolaria. Quartz in the form of subsequent grains of microquartz (<35 μ ) and lesser megaquartz is the only silica phase present. Color varieties of chert differ in fabric of microquartz and content of pigmenting impurities; green chert contains abundant illite; tan, brown, and black chert contain iron and manganese oxides and organic matter; red chert contains hematite; blue chert contains apatite. Mottled chert beds formed by submarine slumping. Novaculite (snow-white chert) owes its lack of color to the absence of detrital impurities and chemical pigments, and its milkiness to the dispersion of light by minute water inclusions (Folk, 1965) and reflecting faces of microgranular quartz crystals. Novaculite has a distinct microscopic fabric; subspherical specks up to 200 μ in diameter composed of microquartz grains up to 10 μ long are scattered in a matrix of slightly larger uneven-grained microquartz grains, 5 to 25 μ long; the specks are probably relict Radiolaria. Skeletal ghosts of spicules visible in ordinary light range from 0 to 80 percent of novaculite beds, but average 10 percent. Radiolaria are more abundant than spicules in colored chert beds. The Caballos was deposited in a deep-marine trough adjacent to peneplaned land masses; rate of accumulation (assuming 50 percent compaction) was from 0.1 to 0.5 mm/1000 years. Terrigenous clay was absent during accumulation of proto-novaculite, but was supplied occasionally during accumulation of colored chert and shale beds. Green and red (originally brown?) colors of clay shale reflect original differences in bottom conditions during deposition. Quartz silt in chert includes wind-blown and storm-deposited grains; sandstone, calcarenite, and conglomerate beds were deposited by rare turbidity currents. Depositional fabrics of sediments were modified by lithification processes, burrowing infauna, escaping gases, and submarine slumping. Chertification occurred largely by solution of opal and precipitation of crystalline silica (cristobalite or low-quartz) from pore solutions relatively soon after deposition.