C. D. Walcott—Pre- Carboniferous Strata in Arizona. 489 
No traces of the Silurian groups have been discovered. At 
the head of Nun-ko-weap valley, where the Devonian is absent, 
species of the genera Lingulepis and Orepicephalus were found 
Within two feet of the base of the Red Wall limestone. On 
examining the few imperfect fossils, collected in 1879 in the 
anab cation, on which the strata just beneath the Devonian 
Were referred provisionally to the Calciferous horizon, they 
were found to be generically identical with species from strata 
lower down in the Tonto that are the geologic equivalents of 
the Potsdam sandstone of the Mississippi valley. i. 
e Tonto is the first of Powell’s great plateau system of 
formations. Of Upper Cambrian age, it terminates below that 
ereat series of comformable deposits that extend upwar 
through over 14,000 feet of strata to the Tertiary Pink Cliffs of 
Southern Utah. It occupies an important position in the to- 
Careous and sandy: strata, the Red Wall cliff is kept clear of 
debris and undermined so as to preserve its mural face above a 
- The latter is wonderfully persistent for a cliff not over 
800 feet in height. Winding in and out, here a bold head- 
land, there notched by a narrow, profound cafion, a mural 
precipice in range for a mile or more, then forming a symmet- 
niecally curved point around which the contours sweep in grace- 
Potsdam sandstone horizon of Central N evada, the Mississippi 
valley and Saratoga county,* New York 
The base line of the Tonto is quite uniform and rests uncon- 
formably on the varied strata beneath ; here and there a knoll, 
Point or ridge is seen on the Pre-Tonto surface that rises nearly 
through the massive Tonto sandstones that were deposit 
against and over them, the sea breaking off, and burying with 
the drifting sand, fragments of the rocky islauds. 
_* The Pot a of Saratoga county occurs in a massive bedded magnesian 
limestone and will be described later. The genera Ptychaspis, Orepicephalus, 
Dicellocephalus, ete., are well represented, 
