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Although the Cheyenne member of the Purgatoire and the Dakota sandstone contain permeable beds wherever found, each is a source of fresh water only where it occurs near the surface: at great depths, the water in the aquifers is highly mineralized. The greatest potential supply occurs generally in the areas where the thickest section of water-bearing units lie within the depth zone of fresh water. In general, fresh water occurs throughout all the water-bearing formations younger than the Dakota. All the previously mentioned formations underlie the center of the Denver Basin where the Denver formation, the youngest unit discussed in this report, is at the surface. Only those above the pierre shale, however, contain potable water. Radially out from the center of the basin, successively older formations are exposed, and the fresh-water-bearing zone above the Pierre becomes thinner. The Pierre appears at the surface to the west within a relatively short distance from the center of the basin: whereas, it appears at the surface to the north, south, and east at some- what greater distances. OVer most of the northern two-thirds of eastern Colorado, the formations above the Pierre shale contain fresh ground water. Where these younger water-bearing deposits above the Pierre shale are thin or absent, ground water is scarce. In the southern one-third of eastern COlorado, the Cheyenne sandstone member of the Purgatoire and Dakota sandstone are the major aquifers, and the younger aquifers are thin or absent. The Dakota and Cheyenne member yield moderate to large quantities of water near the Kansas line, but west of prowers and Baca Counties their potential yields appear to be generally less. In most places, however, the Dakota sandstone is capable of yielding at least enough water to supply domestic and stock needs. In the southeast corner of the state, rocks of TriassiC and JUrassic age contain some fresh water, but the area of occurrence is small. Further exploration may show that they contain fresh water elsewhere in the south- eastern part of the State. West of the Front Range.--West of the Front Range, the prin- cipal aquifers in CO~oradO are of Triassic, JUrassic, or Cretaceous age. Older rocks generally are not aquifers or contain water too mineralized for most uses. The Dakota sandstone is probably the most extensive and best known aquifer in this part of the state. The Mancos shale, a relatively impermeable formation similar in character and age to the Pierre shale in eastern colorado, over- lies the Dakota sandstone and is at or near the surface in large areas in western COlorado. In these areas, wells must penetrate as much as 4,000 feet of material to obtain water from the under- lying aquifers. In some places, the deep penetration may be in vain because the underlying water is salty. 19