The scenic beauty, rugged contour, and great variety of geologic phenomena displayed by the San Juan Mountains of southwestern Colorado have lured to
that region many geologists, both professional and amateur, as well as countless laymen. Serious geologic studies in these mountains seem to fall historically into three periods. The first of these covers the work of the geologists and topographers of the United States Geological and Geographical Survey of the Territories.
Field studies were prosecuted under the general direction of F. V. Hayden in this region from 1869 to 1876, and the results obtained were published among
the monographs of the Hayden Survey. The second period of geologic examinations began in 1887, when Whitman Cross undertook investigations of certain of the mining districts in the San Juan region, which by that time had attracted much attention in the mining world. The results of this second series of studies have
appeared from time to time as folios"' of the Geologic Atlas and reports on the economic geology of the mining districts, published by the United States Geological Survey between 1899 and 1910. Gradually the concept of the geologic problems was changed. Instead of considering individual mining districts as the units of investigation it became apparent that the San Juan region must itself be the unit. In 1908 Mr. Cross drafted plans for the completion of the San Juan studies on this enlarged basis. His aim was to arrange for the publication of papers on different subjects rather than one huge monograph on the region as a whole.
