MERRILL] METEOR CRATER OF CANYON DIABLO, ARIZONA 487 



mospheric fractional heat would ignite and burn away the sulphide portions. 

 Even where the heterogeneous masses of considerable size fall to the earth, 

 it is possible that these susceptible portions would oxidize and wholly disap- 

 pear, leaving the more refractory to be found later. This would account for 

 the almost constant association of shale and irons of the type shown in plate 

 Lxxin at various points out on the plain. 



The occurrence of the still incompletely oxidized forms — shale balls — as 

 described, is due to the protective action of the dry soil in a region of great 

 aridity, the annual precipitation, as recorded by the Weather Bureau, being 

 but about 8 inches. It has been shown that under such conditions soils rarely 

 or never become saturated with moisture for more than a few inches below 

 the surface, and that this moisture is brought back by capillarity and evap- 

 orated rather than drained off at lower levels, as in more humid regions. An 

 iron thus buried, even though rich in chloride and sulphide, would endure for 

 a long period. 



The connection of the shale balls with the embedding material is 

 noted on page 465. 



Analyses of the new variety of the iron and of the shale are given 

 on page 484 ; those of the three iron phosphides 1 and of cohenite have 

 not been reproduced. 



VII. Origin of the Crater 



Opinions for and against the Meteoric Hypothesis. — The origin 

 of the crater has been discussed or suggested from time to time by 

 various authors. So far as I am aware or as shown by the literature, 

 the first hypothesis that need be taken seriously is that put forward 

 by Mr. W. D. Johnson, 2 who concluded from a somewhat superficial 

 study that "in some way, probably by volcanic heat, a body of 

 steam was produced at a depth of some hundreds or thousands of 

 feet, and the explosion of this steam produced the crater." In this 

 view the occurrence of the meteoric irons was of course merely a 

 coincidence. 



Mr. Gilbert, at whose request Mr. Johnson had made the prelim- 

 inary studies, acknowledged himself as not quite satisfied with these 

 conclusions, and in the summer of 1892 undertook, in cooperation 

 with the late Marcus Baker, a more detailed study of the region. In 

 connection with these studies there was prepared by Mr. Baker the 

 topographic map, a copy of which is here reproduced. 3 An excellent 

 series of photographs were also taken, a part of which is here utilized, 



1 Derby thought to find three forms in the typical iron also. 



2 See Gilbert, op. cit., p. 9. 



3 For a redrawing of this map from a reduced photographic copy I am in- 

 debted to the U. S. Geological Survey. 



