On the southern side of the canyon the lower purple stratum shows three distinct substrata; the upper is reddish purple and about 3½ feet thick, the middle is purple gray, about 7 feet thick, and apparently softer than the upper and lower strata. The lodges occur in the middle purple substratum, their floors composed of the upper surface of the lower stratum and their roofs of the under surface of the upper stratum. Those on the north side are similarly placed, their roofs being about 3 feet below the white, except that in several instances the upper part of the purple up to the white has fallen, making the cavity larger. This has occurred, however, since the abandonment of the caves, and the debris, still fresh looking, is in situ. The formation in which the lodges occur is not of volcanic origin, although the beds composing it were perhaps deposited by hot springs during the period of great volcanic activity which produced San Francisco mountain in central Arizona and the great lava flows south of it. In view of the uncertainty on this point and the further fact that almost all the cavate lodges heretofore found were excavated in tufa, ash, or other soft volcanic deposits, the report of Mr. Joseph S. Diller, petrographer of the U.S. Geological Survey, will be of interest. It is as follows: The coarse-grained specimen is sandstone, that of medium grain is argillaceous sandstone, and the fine-grained one is calcareous clay. The coarse-grained friable sandstone, in which the lodges have been excavated, consists chiefly of subangular and rounded grains of quartz and feldspar with a small proportion of black particles. Many of the latter are magnetite, while the others are hornblende and various ferromagnesian silicates. I did not detect any fragments of volcanic origin. The specimen of argillaceous sandstone is made up of thin layers of fine-grained sand of the same sort as the first, alternating with others containing considerable clay. In the clay layers, a trace of carbonate of lime was found here and there, forming a transition of the calcareous clay. The calcareous clay when placed in acid effervesces vigorously, but when allowed to stand the effervescence ceases in a few minutes and the insoluble white clay remains.