216 "W. T. LEE AFTON CRATERS OP NEW MEXICO 



El Paso, and Stanton and Vaughan* later described the section in detail. 

 These rocks are exposed east of the old valley in the Kodadero peak near 

 El Paso, and the writer found fossils near the northern end of the 

 Potrillo mountains, a few miles southwest of the Afton craters, which, 

 according to Stanton, f belong to the Fredericksburg group of the Co- 

 manche series. These rocks in many parts of southern New Mexico and 

 western Texas are underlain by the Permo-Carboniferous red beds,J 

 which are known to contain beds of salt and gypsum. The occurrence 

 of the Comanche beds on either side indicates that the ancient valley is 

 probably eroded either in this formation or through it into the Eed beds. 

 The probable occurrence of salt, gypsum, and limestone beneath the Afton 

 craters will be referred to in considering the ways in which the craters 

 may have been formed. 



rOLGANIG AGTION 



The Afton craters are located in a region of recent volcanic activity. 

 Sheets of basalt cover a considerable area of the plain, which with their 

 associated volcanic cones represent at least two periods of eruption. 

 The older lava, one sheet of which is exposed in the Afton craters, is 

 more or less eroded and partly covered with soil. The younger cones 

 and associated flows have rough, unweathered surfaces and contain caves, 

 open fissures, and other evidence of recent formation. Volcanic cinders 

 are nowhere conspicuous. The cones are low, spreading, and composed 

 almost exclusively of flow lava, little evidence of explosive action being 

 found in connection with them. The flows are not notably scoriaceous 

 and occur in broad, thin sheets, which suggest a very fluid and highly 

 heated condition of the rock at the time of its extrusion. One of the 

 smallest of the recent cones, having at the apex a depression about 100 

 feet in diameter, occurs between the two Afton craters. 



TIME OF ERUPTIONS 



The position of the older lavas above thick beds of Pleistocene sand 

 proves that the time of their extrusion was long after the opening of the 

 Quaternary. Judging from the evident difference in the amount of 

 weathering of the lavas, a considerable length of time intervened between 

 the two periods of eruption, rendering it probable that the last eruption 



* T. W. Stanton and T. Wayland Vaughan : Section of the Cretaceous at El Paso, 

 Texas. Am. Jour. Sci., vol. 1, 1806, p. 21. 



t Personal conamunication. 



t On account of the resemblance of these beds to the Red beds of eastern New Mexico 

 and Texas, they have been frequently referred to as Permo-Trias. While some Permian 

 red beds have been reported from the Rio Grande region, the great bulk of the red sand- 

 stones and shales are now known to be older than the Permian, as shown by the writer 

 in a brief article in the Journal of Geology, vol. 15, 1907, pp. 52-58. 



