894 PROCEEDINGS OF THE ACADEMY OF [DeC, 



imaginable subfeature to a very great degree. This is almost entirely 

 due to the irregularity of its deposition, slightly modified later by the 

 action of water. The surface material of the outside of the rim, where 

 it is not covered with blown sand, as on the southern side, is composed 

 of the broken debris of the three strata through which the hole pene- 

 trates, piled together in the utmost confusion and disorder, pieces from 

 all the three strata being thrown together in the most intimate mixture 

 with a slight tendency towards inversion in the order of their deposi- 

 tion. That is, there is rather more of the red sandstone in the deeper 

 portions of the rim than on the surface, while on the surface the lime- 

 stone and white sandstone predominate, with here and there large 

 areas of unmixed white sandstone lying on the surface. In size these 

 fragments vary from huge rocks forty to fifty feet in length and weigh- 

 ing thousands of tons down to impalpable powder and all intermediate 

 sizes, and many of the rocks are so crushed and broken that they barely 

 hold together. And imbedded in the deposits of impalpable powder 

 are many pieces still retaining the form of rocks, still showing the 

 stratification and bedding planes distinctly, but so crushed as to have 

 lost all solidity. These crushed rocks in many cases have been subjected 

 to such pressure that not only is their consistency as rocks destroyed, 

 but even a certain proportion of the sand grains composing them have 

 been utterly destroyed and they can be rubbed between the fingers to 

 a fine powder, the grains of which will average much less than that of 

 the sand grains originally composing the stone. 



This powder forms a very considerable proportion of the substance 

 of the rim. It is not merely a filling material occupying the interstices 

 between the rocks, as might be a rock pile with fine material water- 

 washed or wind-blown into it until all the crevices were filled up solid. 

 But it occurs in distinct deposits, sometimes alone and entirely free 

 from rock fragments and sometimes mixed with a larger or smaller 

 proportion of rock fragments. When this mixture occurs, the rock 

 fragments are usually so far apart that each rock is entirely surrounded 

 and supported by the powder. Such deposits of powdered rock are 

 often overlaid by a cover of broken rock many feet thick, the individual 

 rocks in places weighing a hundred tons or more. In fact, as far as at 

 present developed, it seems to be a very general feature of the structure 

 of the rim that the lowest material, that lying upon the top of the orig- 

 inal surface, is a greater or less depth of this powdered rock, some- 

 times alone and sometimes mixed with rock fragments, and that on this 

 rests and is supported the whole of the detrital cover which constitutes 

 the crest and outer slopes of the rim. 



