The Sierra Abajo was studied in 1859 by Dr. J. S. Newberry, geologist of the Macomb Expedition, who writes: “Within the last few weeks we have been on three sides of this sierra, and have learned its structure quite definitely. It is a mountain group of no great elevation, its highest point rising some 2,000 feet above the Sage-plain, or perhaps 9,000 feet above the sea. It is composed of several distinct ranges, of which the most westerly one is quite detached from the others. All these ranges, of which there are apparently four, have a trend of about 25° east of north, but being arranged somewhat _en echelon_, the most westerly range reaching farthest north, the principal axis of the group has a northwest and southeast direction. The sierra is composed geologically of an erupted nucleus, mainly a gray or bluish-white trachyte, sometimes becoming a porphyry, surrounded by the upheaved, partially eroded, sedimentary rocks. The Lower Cretaceous sandstones and Middle Cretaceous shales are cut and exposed in all the ravines leading down from it, while nearly the entire thickness of the Cretaceous series is shown in spurs which, in some localities, project from its sides; apparently the remnants of a plateau corresponding to, and once connected with, the Mesa Verde. Whether the Paleozoic rocks are anywhere exposed upon the flanks of the Sierra Abajo I cannot certainly say, though we discovered no traces of them. It is, however, probable that they will be found in some of the deeper ravines, where, as in most of these isolated mountains composed mainly of erupted material, they are doubtless but little disturbed, but are buried beneath the ejected matter which has been thrown up through them. “The relations of the Cretaceous rocks to the igneous nucleus of the Sierra Abajo are very peculiar, for, although we did not make the entire circuit of the mountain mass, and I can, therefore, not speak definitely in regard to the western side, as far as our observations extended we found the sedimentary strata rising on to the trachyte core, as though it had been pushed up through them.” (Geology of the Macomb Expedition, page 100.)