1905.] NATURAL SCIENCES OF PHILADELPHIA, 897 



stone in that order above it. This series naturally suggested the idea 

 that the surface stratum, having received the blow and started on its 

 aerial flight first when the hole was formed, finished its journey first and 

 was consequently deepest imbedded in the silica which was in process 

 of filling the hole made during the flight of these rocks in the air. 

 Almost immediately after the fall of the last of this series — which must 

 have fallen directly in place as found and which is comparatively rare, 

 as the rocks expelled from the hole had usually (apparently) a greater 

 outward radial component in the direction of their flight — came the 

 rush of talus rocks, which fell in masses on the funnel-shaped cliffs 

 surrounding the hole and forming the interior of the rim, and rushing 

 inward covered the surface of the bottom of the hole to a consider- 

 able distance from the foot of the cliffs, in fact probably all except 

 a small area of 300 or 400 feet in diameter in the center. Then, 

 during minutes and hours, settled down over everything about the 

 locality the dense cloud of dust to the depth of many feet. This dust, 

 being the finer portions of the silica above described, was then washed 

 into the center of the hole, filling it in some places a hundred feet deep. 

 This was apparently done by successive wet seasons for many years, 

 during which time, at least in the rainy season, a shallow lake occupied 

 the bottom of the hole; over the bottom of which the sediments were 

 distributed in yearly level strata by wave action. The presence of 

 the rare stone fragments in these sediments and the few now on the 

 surface of the interior plain, far beyond any possible place to which 

 they could have rolled if detached and falling from the cliffs, is difficult 

 of explanation unless it be due to a frozen condition of the central lake, 

 on the surface of which these rocks (and they have not been observed 

 of large size) could slide and on which a very slight initial velocity 

 would take them to their present position, to be there deposited upon 

 the melting of the ice. Ten to fourteen inches of ice was formed on the 

 open water in reservoirs in this locality during the last winter. 



No very exact estimate of the amount of this silica dust washed 

 down from the sides of the hole can be made, as the shape of the original 

 bottom of the hole is unknown. It is irregular and in places the sedi- 

 ments are 100 feet thick, and it covers an area of about 1,800 feet in 

 diameter. Moreover, it evidently fills the interstices of the talus of 

 unknown thickness extending over a much greater area. It can only 

 be said that it is a very large amount, many million tons. It probably 

 covered all of the exterior of the rim to an equal or greater depth, all 

 of which is gone. In fact it seems extremely probable that the rock 

 cover of the rim, which is now its most prominent feature, on the sur- 

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