Eastern Colfax County and northern Union County in northeastern New Mexico contain over 700 square miles of basalt flows which were fissure eruptions chiefly, although extrusion loci are as yet not correlated with post-Eocene regional structure. Physiography indicates three periods of basalt extrusion, separated by active stream erosion. Scoria cones are present on flows of all periods but are most numerous on the intermediate Clayton flows. Lithologic character correlates with the age of extrusion. Olivine basalts predominate in all three periods. Clayton basalts vary most in mineralogy and include olivine basalts, olivine-free basalts, olivine basalts with quartz inclusions, feldspathoid basalts, and basanites. Recent Capulin basalts have the least olivine and the best-developed plagioclase microphenocrysts. Dacites, andesites, soda-trachytes, and phonolites are slightly older than Clayton basalts and definitely younger than the Raton basalts. The dacites are relatively uniform and form steep, eroded cones in eastern Colfax County. Soda-trachytes are rarest and closely bunched in central eastern Colfax County. Phonolites are found in central eastern Colfax County as flows, sills, and dikes. All the Quaternary igneous rocks of the area have probably originated from one magma whose original composition approximated an olivine basalt.