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2. Barker, F.: Precambrian and Tertiary geology of Las Tablas quadrangle, New Mexico. New Mexico Bur. Mines and Min. Res. Bull. 45, 104 (1958).
3. - A brief geologic history of the Precambrian rocks of the Needle Mountains, southwestern Colorado. In: Guidebook of San Juan-San Miguel-La Plata Region, New Mexico and Colorado. New Mexico Geol. Soc. 19th Field Conf. 148?149 (1968).
4. - Precambrian geology of the Needle Mountains, southwestern Colorado. U.S. Geol. Surv. Profess. Paper 644-A (in press).
5. Bickford, M. E., Fred Barker, G. W. Wetherill, and Chin-Nan Lee-Hu: Precambrian Rb-Sr chronology in the Needle Mountains, southwestern Colorado. J. Geophys. Res. 74, 1660?1676 (1968).