864 PROCEEDINGS OF THE ACADEMY OF [DeC, 



Fe Ry., in Coconino county, Arizona, a very remarkable and almost 

 perfectly round crater, differing in many respects, as will be hereafter 

 seen, from any crater on the earth's surface with which I, at least, am 

 familiar. The rocks exposed in this region, and for many miles around 

 in every direction, belong to the Aubrey formation of the Upper Car- 

 boniferous series. These beds are perfectly horizontal, never having 

 been disturbed since they were laid down except by volcanic tremors, 

 which were probably the cause of several small but deep cracks in the 

 vicinity of Canon Diablo gorge and running parallel with it. Erosion 

 has removed the upper strata which overlie these beds elsewhere in the 

 region, so that now the uppermost stratum which is found is red sand- 

 stone, and this exists only as isolated and quite widely separated flat- 

 topped buttes. It is not likely that this stratum was ever of great 

 thickness. The portions of it which are left vary from a few feet to less 

 than fifty feet in thickness. At the place now occupied by Coon 

 Mountain and its crater (for it should be stated that this crater is 

 within a rather low long mountain rising out of the level plain to a 

 height of from 120 to 160 feet, the irregular top of the mountain form- 

 ing the rim of the bowl-shaped crater) this sandstone stratum, at the 

 time the crater was made, probabl}^ existed here as a flat-topped butte 

 of considerable area, not over thirty feet in height aljove the surround- 

 ing limestone plain. The exact locality now occupied by the moun- 

 tain and the crater was no doubt very similar to portions of the present 

 surrounding plain before the event which produced them. These 

 isolated buttes of red sandstone, which are dotted over the plain, 

 probably average from fifteen to twenty feet in thickness. Under- 

 neath this sandstone there are from 200 to 350 feet of yellowish-gray 

 calcareous sandstone, which when eroded and weathered has the ap- 

 pearance of limestone. In fact, this stratum, wdiich is well shown in 

 the neighboring gorge of Canon Diablo, is referred to by the United 

 States Geological Survey as the Aubrey limestone. For the sake of 

 clearness it will hereafter be referred to as limestone. Underneath this 

 limestone there is a stratum of apparently from 800 to 900 feet in 

 thickness, but probably much less,^ of very light gray, almost white, 

 fine-grained sandstone; and underneath this stratum there is a thin 

 stratum of yellow sandstone, the thickness of which is not definitely 



' It is probable that these figures are very excessive and that the true thickness 

 of this sandstone stratum at this point mucli more nearly approximates the 

 thickness given to it in the record of tlie Winona well given l)elow. The record 

 of our bore holes and as. obtained from the surrounding exposures must of 

 necessity be unreliable, for reasons wliich will hereafter appear. 



