190 



N. H. DARTON 



3 53 



o ft 



The origin and history of 

 the depression are an interest- 

 ing problem. From the conti- 

 nuity of its sandstone walls, it 

 clearly is not a portion of an 

 old valley dammed by a lava 

 stream, and it presents none 

 of the ordinary features of a 

 crater. In some respects it is 

 comparable to "Coon Butte," 

 a great depression west of 

 Winslow in eastern Arizona, 

 which, Mr. Gilbert has shown, 

 is due to explosion, but 

 although there is a ring of 

 ejected rocks around the mar- 

 gin of the Zuni depression, the 

 relative bulk is small and the 

 materials are waterlaid. The 

 most reasonable hypothesis 

 appears to be that the depres- 

 sion is due to the sinking of 

 its bottom, and the fact that 

 there are salt springs and 

 the area is underlain by salt- 

 bearing beds are significant 

 in this connection. It seems 

 possible that a salt bed has 

 been dissolved, possibly by 

 hot water issuing from a vol- 

 canic vent, and the depressed 

 area has dropped or faulted 

 several hundred feet. Several 

 instances are known of the 

 development of deep crater- 

 like depressions due to under- 

 ground solution of salt and 



