868 PROCEEDINGS OF THE ACADEMY OF [DeC, 



failed to find a single piece of eruptive or metamorphosed rock, or any 

 -rock indicative of solfateric activity, which has not in all probability 

 been brought to the locality by Indians or the prehistoric inhabitants 

 of this region. 



The sharp edges of the angular fragments of rock, which have cer- 

 tainly been expelled out of this crater with great force, are indicative 

 of the recent origin of the crater. In fact, I am ready to believe that 

 it is not more than 2,000 or 3,000 years old, and perhaps much younger. 

 Cedars have been found growing on the rim which are upwards of 700 

 years old. Were it not for this fact the evidence afforded by the frac- 

 tured surfaces of the rocks would indicate even a more recent origin.^ 



The interior of the crater can best be likened to a great bowl, except- 

 ing that there is an almost vertical escarpment running around the 

 upper portion of the basin, formed of cliffs composed of limestone and 

 the overlying red sandstone. From the bottom of the limestone 

 stratum, or where the more or less shattered and disintegrated white 

 sandstone begins to be seen underneath the limestone cliffs, a great 

 interior fringe of talus commences, which is composed of angular 

 fragments of red sandstone, limestone and gray or white sandstone. 

 This talus slopes at a very low and, for talus representing the effect of 

 weathering, an unusual, or as I think an impossible, angle toward the 

 interior of the plain ; but before it reaches the center it disappears under- 

 neath stratified sedimentary material which was undoubtedly deposited 

 while the interior of the crater was a lake bottom. There are about 

 seventy feet, and perhaps somewhat more in places, of this material, 

 as has been proved by the shafts and drill holes which have penetrated 

 it. It is composed very largely of wind- and water-borne silica or pul- 

 verized sand grains, in which are found numerous fresh-water shells. 

 There are some layers composed almost entirely of microscopic shells, 

 and in some of these sediments there are to be found great numbers of 

 miscroscopic organisms which have silicious skeletons. There has been 

 no opportunity to submit these fresh-water shells and organisms for 

 examination, but it is intended to have this done at an early date. 

 Underneath this sedimentary material there is to be found a more or 

 less [compact and unknown quantity of pulverized sandstone (silica), 

 containing here and there angular rock fragments or so-called boulders. 

 The upper portion of this sedimentary material forms, with the over- 



* It is possible that the cause of this crater may possess considerable historical 

 interest, as explaining the hitherto unexplained fact that throughout this portion 

 of Arizona there are indisputable evidences that the prehistoric civilization 

 ceased abruptly several thousand years ago, according to the necessarily rough 

 estimates of the time which has elapsed. 



