A number of papers have appeared in recent years concerning the Carboniferous formations in the Rio Grande Valley. As the observations therein recorded were almost wholly of a reconnaissance nature, it is not to be wondered at that more or less divergence is manifest in the views of the different writers concerning the stratigraphy of the region. This paper is based on observations made during a year's residence at Socorro, together with several months' field-work in connection with the investigation of the mines and mining districts of New Mexico under the direction of Mr. Waldemar Lindgren of the U. S. Geological Survey. The results of that investigation are to appear in a forthcoming bulletin of the Survey. In the present paper it is the chief purpose of the author to present a brief description of the rocks in the Rio Grande Valley which represent the lower part of the "Coal Measures" of adjoining regions, and to discuss their stratigraphical relations and the nomenclature connected with them. For these beds in the Survey paper above mentioned the name Magdalena has been adopted as a group term.