does not occur in the Rito de los Frijoles is that the hard stratum forming the cap of the cliff is traversed by numerous vertical or nearly vertical cleavage planes, so that it keeps falling to the talus as rapidly as erosion of the main face of the cliff progresses. It is probable, also, that the SW, facing NE, and hence suffering less from heat and consequence evaporation, is somewhat better protected by vegetation, which would serve to hold the debris on the slope and thus aid in preventing the formation of a cliff. This difference In vegetation does not as readily appear to the eye as it does in many similar situations in the semi arid west, but even a slight difference, continued through thousands of years, would exert a great aggregate influence upon erosion. The "tent rocks" or cones at the base of the cliffs have an origin similar to that of the cliffs, protecting cap in such cases being small masses of resistant material instead of continuous, approx-
