1905.] NATURAL SCIENCES OF PHILADELPHIA. 893 



normally be lying level some 150 feet below the surface. These strata 

 themselves clip downward and outward from the center of the hole at 

 an angle of, on the average, about thirty degrees, although this varies 

 in places from more than vertical or inclining backward to about ten 

 degrees. The strata themselves are crushed and shattered to an extra- 

 ordinary degree, and the surface of the rim slopes upward and outward 

 from the center of the hole at an angle of from fifty to eighty degrees ; 

 possibly sixty degrees would describe the general shape better than any 

 other slope. Considering the shattered and disintegrated material of 

 which these cliffs are composed, it is remarkable how little talus has 

 fallen from them. This slope continues up almost to the top of the 

 ridge, although here and there are flat benches in it both at the junction 

 of the yellow limestone and the red sandstone and at partings in the 

 red sandstone itself. From fifteen to forty feet from the top of the 

 ridge on the inside is located the top of the red sandstone, which was 

 the original surface of the plain ; at the place of impact and from this 

 point the ridge slopes outward at the ordinary sliding angle of loose 

 materials, somewhat less than forty degrees, to its summit. The sum- 

 mit of the ridge is of necessity a closed ring and is sharply serrated into 

 peaks, and the colls between these serrations do not exceed thirty to 

 forty feet in depth but their slopes are steep, often ten to twenty de- 

 grees. There is a marked low place in the rim, extending over nearly 

 one-sixth of its circumference on its northern side. On the outside 

 no description will suffice for all sides. The greatest amount, by far, 

 of the material thrown out of the hole is found in the southern quarter 

 of its circumference, and here the rim is almost flat on top for a number 

 of yards and then slopes outward at an angle of only seven degrees for 

 some 900 feet, where it ends in a sharp slope of some twenty-five feet 

 high at an angle of some twenty degrees. Beyond this is a thin cover 

 of ejected material and detached and partly buried limestone frag- 

 ments which extend for a considerable distance; some of the latter 

 having been thrown nearly a mile from the edge of the hole. The 

 actual surface of this southern side of the rim consists largely of blown 

 sand, as the winds in the country are strong and storms frequent and 

 their usual direction is from the southwest. On the eastern, northern 

 and western sides the ridge is thin and sharp ; in many places not over a 

 yard or so in thickness at the very top and sloping outward very 

 sharply, in places up to thirty degrees, for about half its height, and 

 then more gradually at some five degrees until it joins the plain. The 

 general surface of the outer slope is not at all a smooth cone of the 

 angles above stated, but is cut up into hills and hollows and every 



