BULLETIN OP THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 18, pp. 211-220, PLS. 3-4 MAY 16, 1907 



AFTON CRATBES OF SOUTHERN NEW MEXICO* 



BY WILLIS T. LEE 



{Read before the Society December 29, 1906) 



CONTENTS 



General description 211 



Geographic features 212 



Geologic conditions 213 



Quaternary sands 213 



Pre-Quaternary formations 215 



Volcanic action 216 



Time of eruptions 216 



Hydrologic conditions 217 



Other depressions similar to the Afton craters 217 



Possib' causes of formation 218 



Choice of hypotheses 218 



Conclusion 220 



General Description 



During the summer of 1904, while engaged in geologic inrestigations 

 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, separ- 

 rated 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 th-e 

 Southern Pacific railroad. The smaller, which is nearly circular in out- 

 line, 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 114 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 a mile or more and rising 10 to 200 feet above 

 the plain. The maximum difference in elevation between the bottom of 



* PubUshed by permission of the Director of the U. S. Geological Survey. Mami- 

 script received by the Secretary of the Society March 2, 1907. 



XVI — Bull. Gbol. Soc. Am., Vol. 18, 1906 (211) 



