256 
THE GEAKD CANON DISTRICT. 
sion in the medium -hard cherty limestones. But this condition is gradually 
brought to a check by the formation of a talus. We have observed that 
the softer and more yielding the beds under the action of weathering the 
longer and less steep will be the weathered slope across their edges. Upon 
this slope across the edges of the shaly sa;ndstones accumulate many of the 
cherty nodules and fragments of the adamantine sandstone, with large 
quantities of fine sand, and even a little soil. The debris lodging there 
protects, to an important extent, the shaly sandstones and retards more and 
more their rate of weathering, retards the rate of undermining, and dimin- 
ishes gradually the supply of debris to the talus. Thus the great increase 
in the rate of weathering caused by corrasion penetrating the yielding shales 
is, to a great extent, countervailed by the formation of the talus. It now 
becomes apparent that the resulting profile of the entire cliff has a perfectly 
definite and stable configuration or typical form, which the combined action 
of all the incident forces tends strongly to maintain. 
The cliff formed out of the upper and lower Aubrey series is very re- 
markable for the constancy of its profile throughout the entire extent of the 
great chasm. Along every mile of the main facade, in every amphitheater 
and alcove, and in every promontory, wing wall, and gable, it discloses the 
same familiar features. It is the wall of the upper or outer chasm. In the 
Kanab, Uinkaret, and Sheavwits divisions it stands far away from the lower 
or inner chasm — an interval of two miles usually separating its base from 
the brink of the inner gorge. In the Kaibab division the Aubrey wall is 
unchanged in its general character, but everything below it is there in strong 
contrast with what is disclosed in the other divisions. In the three western 
divisions the broad platform at the base of the upper wall is a very striking 
feature ; in the Kaibab the platform is quarried away by the lower depths 
of the amphitheaters, leaving only intervening buttes, and the profile at 
the base of the great lower Aubrey talus at once plunges vertically down 
the precipices of the Red Wall limestone. In many places the Aubrey cliff 
in its recession by waste is so closely pursued by the recession of the Red 
Wall below that a few hundred feet of the lower Aubrey talus are cut off 
and the shales at the base of that series are undermined, forming cliffs which 
are continuous vertically with the great Red Wall precipice below. Almost 
