on a peculiar form of metamorphism in sili- 

 cp:ous sandstone. 



By George P. Merrill, 



Head Curalor, Dejxn-tiiieiit uf (icologii, U. S. Naliomd Mtiseiun. 



It will be remembered that in 18!>1 attention was called to a crater- 

 like depression in unaltered sedimentary rocks some twelve miles 

 southeast of Canyon Diablo, Arizona, by the finding- of a large num- 

 l)er of masses of meteoric iron in the immediate vicinity. Subse- 

 quenth' the possible origin of the depression, or crater, was made a 

 matter of investigation l)y Mr. G. K. Gilbert, of the U. S. Geological 

 Survey, who gave his results in his presidential address before the 

 Geological Society of Washington, in 1806, under the caption of The 

 Origin of Hj^potheses. 



With the question of the origin of the crater the present paper has 

 little to do. It is sutlicient to say that Mr. Gilbert, after discus- 

 sing various h3'potheses, was led to regard that of an origin through 

 explosive volcanic action as most plausible. This view has recently 

 been discussed by Messrs. D. M. Barringer and B. C. Tilghman, of 

 Philadelphia," who have undertaken a series of investigations based 

 on the theory that the depression is due to the impact of a gigantic 

 meteorite. 



As is well knowr,, the surface rock over a large part of the region is an 

 arenaceous limestone, known as the Aubrey limestone, which has, ac- 

 cording to Mr. Barringer, a thickness of some o5() feet. Immediately 

 underljnng this is a light gray sandstone from 450 to 500 feet in thick- 

 ness. A peculiar and apparently very local form of metamorphism of 

 this rock is the excuse for the present paper. 



The sandstone (Cat. No. 70884 U.S.N.M.) in its original and pre- 

 vailing type is of a light graN' color, distinctly saccharoidal and, in the 

 walls of the crater, ver}^ friable, being in small masses easily disinte 

 orated in the hands. Under the microscope it is found to be composed 



"Coon Mountain and its Crater, Proc. Acad. Nat. Soi. Phila., December, 1905. 

 1 -lied March 1, 1906. 



Proceedings U. S. National Museum, Vol. XXXII— No. 1546. 



547 



