Report upon United States geological surveys west of the one hundredth meridian, Volume III: Supplement -- geology

The region examined during the seasons of 1878 and 1879 extends north to north latitude 37° 20, and embraces parts of North Central New Mexico, and South Central Colorado. It lies wholly east from the canon of the Rio Grande, includes the mountain area of the Spanish ranges to their southern termination, and takes in the eastern plains to west longitude 104° 7' 30". But of this region a strip between the Rio Grande and the mountains, lying south from north latitude 36° 40" was not visited; and the total area colored on the maps is not far from 10,000 square miles.