1905.] natural sciences of philadelphia. 895 



The Interior of the Hole. 



From the point on the level with the exterior plain on the inside of 

 the rim the walls of the hole slope downward and inwa d at a constantly 

 diminishing angle for a distance varying from 50 to 150 feet, in the same 

 formation as above described as the base of the inside of the rim. At 

 this point the rock walls begin to be covered with a rocky talus corre- 

 sponding in all respects with the rocky cover on the exterior of the ridge. 

 For about half the circumference of the hole the yellow limestone 

 extends downward to the talus, and for the remaining half it exposes 

 more or less of the whitish sandstone below. The white sandstone is 

 a much weaker rock than the yellow limestone, and at their contact it 

 is noticed that the former is much crushed and disintegrated by the 

 pressure exerted b}^ it in lifting the limestone. This stratum of crushed 

 sandstone varies in thickness up to some ten or fifteen feet as a maxi- 

 mum, and in some places, usually immediately below the limestone, it 

 is reduced to a bed of sand grains absolutely unconnected with each 

 other, and in places a small proportion of even the sand grains have 

 been crushed and broken to fragments and powder. 



The very top of the talus slope is in places at an angle of forty degrees, 

 but usually much flatter down to thirty and twenty-five degrees, this 

 rapidly becoming less and less as it recedes from the cliffs until it is 

 lying at an angle of not more than six degrees at the point where it 

 disappears under the central plain. This central plain is an almost 

 circular area of about 1,800 feet in mean diameter, with a surface gener- 

 ally flat but gently rolling within a limit of fifteen feet, with its lowest 

 point a few feet to the east of the central meridian of the hole and about 

 sixty feet south of the center. Shafts have shown the rocky talus to 

 extend under this central plain at about the same angle that it has 

 above for a distance of at least 400 feet, at which point it is some 

 forty-seven feet below the surface and about twenty feet thick. This 

 talus does not extend entirely across the hole. It is absent at points 

 50 feet southwest and 200 feet southeast of the center of the hole. 

 Exactly where it terminates is not known. 



The Silica. 



It is here necessary to describe more minutely the material of the 

 filling of the central plain. This is identical with the impalpably 

 powdered rock referred to briefly above in the description of the rim. 

 This material, of which there are millions of tons in the rim and the 

 bottom of the hole, consists of the rock of the strata concerned reduced 



