220 W. T. LEE APTON CRATERS OF NEW MEXICO 



The hypothesis (4) of caving due to solution of underlying beds, 

 while theoretically possible, finds little more support in observed facts 

 than the one of sinking caused by the removal of underlying volcanic 

 rock. Probably the strongest points that might be urged in its favor 

 are: First, the volumetric discrepancy between the rims and the crater 

 depressions, and, second, the probable occurrence beneath the plain of 

 the salt, gypsum, and limestone deposits of the red beds, the subter- 

 ranean solution of which has elsewhere produced notable sinks. The 

 first has been sufficiently discussed, but further attention should be given 

 to the second. 



Montezumas well, in central Arizona, is presumably due to the solu- 

 tion of underlying limestone, as explained by Blake in the article pre- 

 viously cited. Salt well, in southern Nevada, and to some extent at 

 least the crater of Zuni Salt lake of western New Mexico, are explained 

 by the geologists who have described them as occasioned by the solution 

 of underlying beds of salt. The removal of gypsum by subterranean 

 solution is also Imown to have produced remarkable sinks, notably in 

 Pecos valley, eastern New Mexico, as described by Fisher* and by Lee.f 



Opposed to the hypothesis of subterranean solution, however, is the 

 probable depth of the salt and gypsum. As previously described, the 

 sands of the valley filling are at least 1,000 feet thick and probably rest 

 on Lower Cretaceous strata of iinloiown thiclmess. The nearest out- 

 crops of the Eed beds indicate that 1,000 feet or more of strata intervene 

 between the salt and g}rps-ain horizons and the overlying Cretaceous beds. 

 It is probable, therefore, that the salt and gypsum, if present beneath 

 La Mesa, lie too deep for their solution to have been effective in forming 

 the Afton craters. 



Conclusion 



The observed facts all point to the hypothesis of volcanic explosion as 

 giving the most rational explanation of the Afton craters. It is further- 

 more probable that the explosions were caused by the formation of steam 

 generated by lava that was forced into the water-saturated sands, but that 

 failed to reach the surface. The probable presence of this lava, still re- 

 taining its heat in some measure, is indicated by the occurrence of the 

 warm well-water in the Kilbum crater. 



• C. A. Plsher : Geology and underground waters of the Roswell artesian area, New 

 Mexico. Water-supply and Irrigation Paper no. 158, U. S. Geological Survey, 1906. 



t W. T. Lee : Gypsum beds and water storage in the Pecos valley of New Mexico. 

 Science, new series, vol. 23, 1906, p. 306. 



