500 H. L. FAIRCHILD ORIGIN OF METEOR CRATER, ARIZONA 



the silica and on the plain. The concentric lamination of the oxides has 

 led the explorers to call the material "iron shale." In the hydration 

 process it is apparent that large quantities of the material have entirely 

 disappeared, but innumerable fragments of dark brown or black color are 

 found which are sufficiently compact to take a polish, and great numbers 

 of small unit masses are found which have a fissured or septarium-like 

 surface apparently due to expansion by hydration (see plate 56). 



In color the oxides range from black to yellow, and the decomposing 

 material frequently shows green discoloration due to the nickel. Some 

 fragments are magnetic, but they all are hydrous and seem to be limonite, 

 with perhaps some turgite. 



The metallic centers of the "shale balls" polish and etch as readily as 

 the Canyon Diablo irons, but the crystalline structure is not. quite the 

 same, and on exposure they behave differently. The Canyon Diablo 

 retain polished surfaces indefinitely, but the core irons rapidly oxidize in 

 the atmosphere of the laboratory and exude drops of iron chloride, soon 

 giving a fresh coating of rust. 



It is evident that we have in association with the crater two varieties of 

 siderites, the well-known Canyon Diablo and the chlorine-rich or oxidizing 

 irons. (For brevity the initials C. D. will hereafter be used for the 

 Canyon Diablo irons and C. D. 0. for the oxidizing or decomposing 

 irons.) This is a remarkable and critical fact. Do they represent two 

 distinct falls or are they only variations in the decomposition of one 

 huge meteor? It should also be noted in this connection that two forms 

 of the iron oxides may be distinguished, the limonitic and perishable 

 coatings found on the surface of the oxidizing irons, and the firm, com- 

 pact, and more permanent form, represented by the fissured nodules. 

 Probably these oxides shade into each other, but nevertheless they must 

 indicate variations in the composition of the meteoric material or differ- 

 ent circumstances during the hydration process. 



Eock fragments are frequently found cemented together or even en- 

 closed by the meteoric iron oxides, some of the dark and lustrous masses 

 of the latter suggesting fusion, though the form is from solution. The 

 explorers have raised the question if some of the oxide material might 

 not represent the superficial burning of the meteor in its heated flight 

 through the atmosphere, subsequently changed by hydration. Theo- 

 retically the fused material on the surface of the flying meteorite is 

 swept away as rapidly as formed, but the particles of oxidized metals 

 which formed the incandescent train of the meteor should be found; and 

 the explorers do find a magnetic dust over the plain, over and through the 

 rock debris, and in the crater drillings to the depth of 650 feet below the 



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