mekkiia] ..n.Ti.ok CRATER OS CANYOH DIABLO, ARIZO 473 



all gradations, from that just described to the white, almost dust- 

 like powder designated as "silica" by Messrs. JJarringer and Tilgh- 

 man, but which the writer of this paper will refer to as rock-flour. 

 This, interspersed with more or less firm material, occupies a large 

 portion of the crater from the 85- or 90 foot level down to the 

 underlying red bed, a distance in round numbers of 500 feet. It is 

 found also wherever pits have been sunk on the exterior margin of 

 the rim and in deposits comprising the thousands of tons of material 

 shown in figure 1 of plates i.xvn and j.xviji. Between the thumb 

 and fingers thi material, notwithstanding its fineness, has a sharp 

 gritty feeling, and under the microscope to be eomposed 



wholly of the sharply angular bits of quartz derived from the shat- 

 tering of the individual grains of sand (see fig. \2&). It has been 

 unquestionably derived from the sandstone, and that, too, not by 

 simple disintegration, but through some dynamic agency acting like 

 a sharp and tremendously powerful blow. 



Commingled with this material in the bottom of the crater (as 

 shown by shafts), and to a less extent around the margin and in 

 scattered masses outside of the rim, are fragments of what is 

 plainly sandstone, but of an almost snow-white color, and so friable 

 as .to be readily crushed between the thumb and fingers Hocally 

 known as ;jJiost sandstone). With these, but less abundant, are more 

 compact, but platy forms, almost de-void of appreciable granular 

 structure, but which were yet recognized by Messrs. Barringer and 

 Tilghman as sandstone derivatives, and. more rarely yet, some 

 coarsel}' and finely pumiceous forms, the exact nature of which was 

 uncertain. These last were examined by Mr. Diller and reported 

 upon to Mr. Gilbert at the time he was making his studies, but the 

 results were not published. 



The following is an amended description of these crushed and 

 otherwise altered forms as given by the author in his paper above 

 referred to : 



The sandstone (Tat. No. 76,834, U. S. X. M.) in its original and 

 prevailing type is of a light-gray color, distinctly saccharoidal and, 

 in the walls of the crater, very friable, being in small masses easily 

 disintegrated in the hands. Under the microscope it is found to 

 be composer] of well-rounded quartz granules, with an occasional 

 grain of a plagioclase feldspar, and a little dust-like material in the 

 interstices, but the amount of interstitial material of any kind is 

 very small. The general structure of the stone is shown in figure 

 1, plate lxxi. This type passes into what may be called the first 

 phase of the metamorphism, an almost chalky white rock (Cat. No. 



