470 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL- 50 



IV. The Crater Floor 



Physiography. — As already noted, the crater has at present a 

 maximum depth of 600 feet, measured from the crest of the rim, or 

 about 440 feet below the level of the plain. Beyond the fringing reef 

 of talus the floor presents a nearly level plain of over 300 acres in ex- 

 tent, surrounded on all sides by well-nigh inaccessible cliffs (see pis. 

 Lxix and lxx). It needs but a glance, however, to show that a large 

 amount of material has fallen from the interior walls through the 

 action of gravity, water, and frost (pi. lxvih, fig. 2), and that the 

 original depth must have been considerably greater. How much 

 greater could be only guessed at until the borings incident to the 

 development work of Messrs. Barringer and Tilghman were under- 

 taken. A number of drill-holes have now been sunk to depths up 

 to 1,100 feet, and from the results thus obtained we are enabled 

 to gain a record entirely inaccessible at the time Mr. Gilbert made 

 his studies, and which throws such light upon the subject as to 

 justify us in reverting once more to the original hypothesis, as set 

 out in Mr. Gilbert's paper and advocated by Messrs. Barringer and 

 Tilghman- — that of an origin through impact of a giant meteorite. 



Results of Borings. — Below are given the results of one of these 

 borings (hole No. 17), situated 600 feet south, 84 east (true) of 

 the center of the crater and starting on a surface 540 feet below the 

 rim, or 400 feet below the level of the plain. (See fig. 125.) 



Feet. 



(1) Surface material, soil, sand, and wash from cliffs o- 27 



(2) Lake-bed formations, lying horizontally and containing diatoms, 



shells of mollusks, and abundant gypsum crystals 27- 88 



(3) A sand which gives reaction for nickel and iron and contains 



fragments of metamorphosed sandstone, sandstone pumice, etc. 85-220 



(4) Sand and rock, sand grains crushed slightly, if any, and not 



metamorphosed, barren of meteoric material 220-520 



(5) Sand and "silica" (rock-flour), with abundant slag-like material 



containing iron and nickel, and metamorphosed sandstone 520-600 



(6) Fine silica powder (rock-flour) and sand, no meteoric material. . 600-620 



(7) Bed-rock, a grayish sandstone rapidly becoming yellow and harder, 



not metamorphosed 620-720 



A less detailed record of hole No. 12 is as follows : 



(1) Surface soil, blown sand, etc 0-30 



(2) Lake-bed deposits 30- 90 



(3) Sand (rock-flour), sandstone in part metamorphosed 00-630 



(4) Rock, at first soft and shattered, but becoming gradually harder 



as greater depths were reached 630-830 



