This atlas is composed of two series of maps: the first, of four sheets, on a scale of twelve miles to one inch, each covering the whole State of Colorado; the second, of twelve sheets (six topographical and six geological, of identical areas), on a scale of four miles to one inch, each sheet embracing two and one-half degrees of Longitude and one and one-quarter degrees of Latitude, the whole presenting the results of the field work of 1873, '74, '75 and '76, covering the entire State of Colorado and adjacent portions of Utah, Arizona and New Mexico.