[18]The Geological Survey has divided the Precambrian into, from oldest to youngest, the Archean and Proterozoic Eons, with the boundary at 2,500 million years. The two eons now constitute Precambrian time. [19]Potter and Rossman, 1977. [20]According to Robert A. Cadigan, U.S. Geological Survey. [21]See Pipiringos and O’Sullivan, 1975. [22]See Wright, Shawe, and Lohman, 1962. [23]Pipiringos and O’Sullivan, 1975. [24]According to Fred Peterson, U.S. Geological Survey. [25]Wright, Shawe, and Lohman, 1962; Lohman, 1975. [26]Paul L. Williams, U.S. Geological Survey, told me in November 1978 that he observed H₂S-bearing spring water leaching the color from both the Entrada and Wingate Sandstones above Tabeguache Creek some 8 or 10 miles north of Nucla, Colo., and from the Wingate alone along Onion Creek in eastern Utah. [27]Lohman, 1965a, p. 47, 48. [28]Study of a fauna of small vertebrates from the late Jurassic Morrison Formation of western Colorado, by George C. Callison, 21 pages, June 1978. [29]_A_, modified from Gilmore, courtesy U.S. National Museum; _B_, modified from Gilmore, courtesy Carnegie Museum; _C_, modified from Mathew, courtesy American Museum of Natural History; and _D_, modified from Romer, after Marsh and Gilmore, courtesy University of Chicago Press. [30]From information obtained in several discussions with John R. Donnell. U.S. Geological Survey. [31]See Cater, 1970, p. 67. [32]Lohman 1961, 1965a, 1965b; Hunt 1969, p. 87. [33]Lohman, 1960, p. 66-100, 88-91. [34]In the following sections the small numbered triangles in the right margin refer to key points along the trip routes. Figure 3 shows the locations of these key points. These numbers should be especially helpful if the reader happens to follow the road guides in reverse. [35]One cfs, or one ft³s⁻¹, = 448.8 gallons a minute or 2446.6 cubic meters a day. [36]Lohman, 1965a, p. 44. [37]Lohman, 1965a, p. 22. [38]Lohman, 1963, 1965a. [39]Lohman, 1965a, p. 22, 23. [40]Lohman, 1965a, p. 106, well 33. [41]Lohman, 1963; 1965a, pl. 1. [42]Lohman, 1965a, p. 15. [43]Lohman, 1965a. [44]Lohman, 1965b. _Index_ Italic page numbers indicate major references or pre-eminent views Page