During the summer of 1904, while engaged in geologic investigations in the Rio Grande valley in New Mexico, the writer visited the volcanic region about 30 miles northwest of El Paso and there examined the two depressions locally known as the Afton craters. These depressions, separated by about 2 miles, occur in the level plain, known as La Mesa, west if the Rio Grande and about 8 miles south of Afton, a station on the Southern Pacific railroad. The smaller, which is nearly circular in outline, is about a mile in diameter and has a depth of 150 feet below the general level of the plain. The larger is oblong, 2 miles in length and about 1¼ miles wide, with a depth of 250 feet below the surface of the plain. Each depression is surrounded by a rim varying in width from a few hundred feet to half . . .