498 H. L. FAIRCHILD ORIGIN OP METEOR CRATER^, ARIZONA 



54) ; but the silica is evei^Avliere encountered in the pits and trenches. 

 Fragments of the rocks of all sizes are scattered irregularly through the 

 silica, but quite clean sections of the powder are found up to 40 feet deep ; 

 and it is the predominating material in contact with the upturned red 

 sandstone. On the south side of the crater rim the silica is exposed in a 

 ravine some 600 feet long and 8 to 10 feet deep. Imbedded in the silica 

 are not only quantities of the limestone and the red sandstone, but also 

 masses of the white sandstone in all stages of the crushing. Some of the 

 blocks preserve the bedding planes and oblique lamination and to the eye 

 appear as firm, unaltered rock, but under a little pressure crumble in the 

 hand into the flour-like silica. By the explorers these have been aptly 

 called the "ghost" sandstone. 



Two metamorphic forms of the sandstone are found which are of spe- 

 cial significance. One form ("variety A") has a higher density with a 

 slaty structure, the cleavage having no relation to the rock lamination. 

 This form seems to be a direct product of intense compression. It is 

 found in the drill holes to the depth of over 400 feet. 



The other peculiar form ("variety B") of the transformed sandstone is 

 a very light, cellular or pumiceous structure, of considerable firmness and 

 rigidity, but so light that it will float on water like a piece of wood. This 

 pumiceous form is rare, but is reported from the drillings to the depth of 

 160 feet below the sediments. Under the microscope it is foimd to be 

 mainly composed of amorphous silica, but with scattered particles of 

 crystalline quartz. It would seem to be the natural product of the in- 

 tense heat of compression on portions of the sandstone which contained 

 sufficient water, perhaps along the joints of the strata, to effect an aqueo- 

 igneous fusion. 



These three forms of the crushed sandstone, the powdery, slaty, and 

 cellular, are the products which should be expected from a tremendous 

 crushing blow with resulting heat. They are not the products of an ex- 

 plosion, which Avould shatter the rocks, but woidd not reduce them to dust. 

 The first two forms are also entirely opposed to the hypothesis of aqueo- 

 igneous origin of the crater, and the cellular form is probably unknown 

 in hot springs and fi;marolic deposits.* 



* The mechanics of impact by projectiles, as relating to the origin of the sandstone 

 powder, is well stated hy Mr. Tilghman as follows (pages 890-890) : 



"To account for the presence of this silica powder on the theory that the hole was 

 formed by a great pro.iectile requires a short preliminary study as to the yielding of 

 hard, brittle, and practically incompressible material before a projectile or other blow, 

 or even quiet pressure, for the method is much the same in both cases. Briefly, the way 

 In which substances yield to either pressure or blow in excess of their poAver of resist- 

 ance is that a cone of material, with an apex angle of about 90 degrees, is compressed 

 downward into the solid mass of the material from -the point of impact. This cone parts 

 from the overlying material, crushes into powder under the force of the pressure or 



