438 C.D. Walcott—Pre-Carboniferous Strata m Arizona. 
One of the strongest topographical features of the Kaibab. 
division of the Grand Cafion of the Colorado and the lateral 
cafions opening into it is formed by the basal member of the 
Carboniferous group, the massive Red Wall limestone of Gil- 
bert. Breaking abruptly off at the foot of the terraced slope 
of the Lower Aubrey sandstone, a sheer cliff of from 800 feet 
at the head of the Grand Cafion, to 1,000 feet at the mouth of 
the Kanab Cafion, is entirely formed of this limestone. Fre- 
quently several hundred feet are added to the cliff by the 
abruptly breaking away of the thin Devonian series and the 
massive Upper Tonto (Cambrian) beds beneath. 
In the field work of 1879 a line of demarcation was estab- 
lished at the base of the Red Wall limestone, and the presence 
of the Devonian between it and the Tonto group of Gilbert 
definitely determined on both stratigraphic and paleontologic 
evidence. A plane of unconformity by erosion, not dip, was 
found between the Carboniferous and Devonian, and also a 
strongly marked fauna compared with that of the Tonto be- 
neath and Carboniferous above.* 
This horizon was traced, during the winter of 1882-3, from 
the most northern exposure in Marble Cafion, where the base 
of the Red Wall limestone first rises above the river-bed, to @ 
point in the Grand Cafion south of Vishnu’s Temple. rom 
the known structure of the cafion walls, this line is undoubtedly, 
present the entire length of the cafion to its termination at the 
rand Wash. It is known in and at the mouth of the Kanab 
Cafion, and Mr. Gilbert’s section indicates its presence at the 
In the Kanab Cafion a strong line of erosion was observed e 
the base of the Devonian. Usually this is very slight an 
* This Journal, IIT, 1880, vol. xx, p. 221. 
