218 W.'t. lee AFTON CRATERS OF NEW MEXICO 



Possible Causes of Formation 



The principal hypotheses that have been developed to explain the 

 various depressions above referred to are as follows: 



1. The impact of a meteorite, originally suggested by Gilbert* and re- 

 cently urged by Barringerf and Tilghman.J 



2. Explosion of steam or other gases, due to volcanic action. 



3. Sinking of the surface, caused by the removal of underlying ma- 

 terial by volcanic action, as in the case of calderas and crater rings. 



4. Caving, of the surface, due to the removal by solution of underlying 

 material. 



Choice of Hypotheses 



Since no indication of the presence of an extra-terrestrial body has 

 been found near the Afton craters, the first hypothesis can be dismissed 

 without discussion. 



The hypothesis of volcanic explosion (2) apparently explains most 

 satisfactorily the observed facts. It is conceivable that explosions may 

 have been caused by the sudden conversion of water into steam, following 

 the entrance of molten lava into the water-saturated sands. The pres- 

 ence of extensive lava flows and large crater cones in the vicinity of the 

 Afton craters shows that great quantities of extruded lava reached the 

 surface in places. The cones vary greatly in size, and the smallest of 

 them, in which only a slight quantity of lava reached the surface, is 

 situated between the Afton craters. It is conceivable that in the case of 

 the Afton craters lava was thrust into the water-bearing sands, but failed 

 for some reason to reach the surface. In other words, these craters may 

 represent one of the first stages of volcanic action in this district. It is 

 probable that at one time the other craters presented some such aspect as 

 the Afton craters now show, and the later eruptions filled the depressions 

 with lava and finally built volcanic cones. 



The composition of the crater rims proves beyond reasonable doubt 

 that the material came from beneath. The supporting facts already 

 stated may be summarized as follows : The rims are composed of an inti- 

 mate mixture of fine sand, volcanic cinders, and blocks of lava, the vol- 

 ume of sand greatly exceeding that of the cinders. The cinders are 

 highly scoriaceous, thus differing notably from the older compact flows — 

 a fact apparently unfavorable to the supposition that the cinders may 



* G. K. Gilbert : Science, new series, vol. 3, 1896, p. 1. 



t D. M. Barringer : Coon mountain and its crater. Proc. Acad. Nat. Sci., Philadel- 

 phia, 1905, pp. 861-886. 



} B. C. Tllghman : Coon butte, Arizona. Proc. Acad. Nat. Sci., Philadelphia, 1905, 

 pp. 887-914. 



