File:Annual report. 1st-12th, 1867-1878 (1867) (14743165626).jpg

Identifier: annualreport1st110geol (find matches)
Title: Annual report. 1st-12th, 1867-1878
Year: 1867 (1860s)
Authors: Geological and Geographical Survey of the Territories (U.S.) United States. General Land Office United States. Dept. of the Interior
Subjects: Geology
Publisher: Washington, Govt. Print. Off.
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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ed irregularly above, and generally quite shal-low, being governed very much in contour and depth by the quality ofthe rock. The work of excavation has not been an extremely greatone, even with the imperfect implements that must have been used, asthe shale is for the most part soft and friable. A hard stratum served as a floor, and projecting in many places madea narrow platform by which the inhabitants were enabled to pass alongfrom one house to another. Small fragments of mortar still adhered to the firmer parts of thewalls, from which it is inferred that they were at one time plastered. Itis also extremely probable that they were walled up in front and fur-nished with doors and windows, yet no fragment of wall has been pre-served. Indeed, so great has been the erosion that many of the caveshave, been almost obliterated, and are now not deep enough to giveshelter to a bird or bat. This circumstance should be considered in reference to its bearing V. S. Geological Surrey. Plate XXXI.
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holmes.) CAVE-TOWN, EIO SAN JUAN. 389 upon the question of antiquity. If we suppose the recess to be destroyedis six feet deep, the entire cliff must recede that number of feet in orderto accomplish it. If the rock were all of the friable quality of the mid-dle part, this would indeed be the matter of a very few decades; but itshould be remembered that the upper third of the cliff-face is composedof beds of comparatively hard rocks, sandstones, and indurated shales.It should also be noted still further that at the base of the cliff thjere isan almost total absence of debris, or fallen rock, or even of an ordinarytalus of earth, so that the period that has elapsed since these houseswere deserted must equal the time taken to undermine and break downthe six feet of solid rock, plus the time required to reduce this mass ofrock to dust; considering also that the erosive agents are here unusuallyweak, the resulting period would certainly not be inconsiderable. Figure 2 gives a fair represen

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