Coffin reported in 1921 that the area mapped by this report includes the well-known shallow carnotite districts of southwestern Colorado, together with certain extended portions of these districts, some of which are believed to be well worth exploring. This does not mean that this area includes all the ground that should be explored. The McElmo formation, which exhibits carnotite in places, extends into Utah, and for long distances north and east of the districts mapped in Colorado. It is entirely possible that Colorado deposits will be found outside this area.
Objects and Scope of the Mapping Survey
The objects of the survey were: To prepare a general base map of the region; to determine and map the boundaries of the McElmo formation in which all the commercial deposits of carnotite have thus far been found, and to prepare a report on the district to serve as a guide in prospecting and an aid in the development of the mineral resources 'of the region.
The original plan was to prepare a suitable base map and to show only the boundaries of the McElmo formation. But as the field work progressed it soon became apparent that it would be possible to map some of the overlying and underlying formations with very little extra work. Accordingly, the plans were changed to allow the mapping of the 'Dakota,' and Mancos above, and the La Plata, Dolores, and older formations below the McElmo. This new plan was carried out only in part, for problems appeared with regard to the division of the formations below the La Plata, which could not be settled in the limited time available. It seemed advisable to adopt arbitrarily a boundary which would indicate the upper limit of the carnotite-bearing horizons, and to map some of the formations together. These arbitrary b01mdaries, with reasons for their adoption, are fully discussed in the chapter on general geology.
[The report includes great maps with cross sections of interpreted geology at depth. See page 259]
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