The igneous rocks described in this paper were collected in an area about 86 by 50 miles in northern Union and eastern Colfax Counties, northeasternmost New Mexico. Lava-capped mesas and volcanic cones form prominent topographic features. The rocks are classified on a mineralogic basis. Basalts are the most widespread, dacites and andesites occur in subordinate amounts, and alkaline rocks such as phonolites and soda trachytes are localized in about 24 square miles in central eastern Colfax County. Petrology substantiates three main periods of basaltic extrusion which physiography indicates: Raton (earliest), Clayton (intermediate), and Capulin (recent). Olivine basalts predominate in all three periods and are mineralogically similar with textural variations. Raton basalts are the most uniform in texture and mineralogy. Clayton flows are predominantly olivine basalts but include olivine basalts with quartz inclusions, analcime basanite, nepheline basalts, haüyne basalt, and olivine-free basalts. Red Mountain dacites have built volcanic cones and occur as plugs and necks. The alkaline suite is sodic. Chico phonolites occur as flows chiefly; Slagle trachytes associated with tinguaites and analcime microfoyaites occur in smaller amounts than the phonolites and are usually intrusives. The sodic rocks fall in line with the other alkaline groups which occur along the eastern front of the Rocky Mountains from British Columbia to Mexico. Dike rocks include leucocratic varieties such as hornblende dacite, phonolite, tinguaite, and analcime microfoyaite; and melanocratic varieties such as olivine basalt and lamprophyres (vogesite and monchiquite). The diverse rock types are believed to have originated from a parent olivine basalt magma.

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