278 The American Geologist. November, 1903. 
era and for a long time prior to the deposition of the Tonto 
sediments. 
The Tonto Formation. — This formation is composed of 
coarse to fine grained, often cross-bedded vitreous sandstones 
and sandy shales, varying in color from brown to red, purple, 
and white. The formation is the surface rock of a large area. 
The Sierra Ancha mountains are capped with it. It is ex- 
posed all around the rim of the Tonto-Cherry Creek basin and 
continues on the east from that basin beyond the head waters 
of Oak creek in the latitude of Chiddessky on Canyon creek. 
From this point it extends in an ever-widening strip along the 
Plateau south across Salt river to the Apache and Nantan 
mountains. Patches of it also occur in the Apache and Pinal 
mountains. In short the Tonto exposures completely encircle 
the Archaean and Algonkian areas of the Salt river and Canyon 
creek region. That it once covered the entire Archaean and Al- 
gonkian area is attested by the vitreous sandstone points and 
buttes scattered over the country, among which are points Chid- 
desche and the Twin and Sombrero buttes. A further mention 
of these buttes and of the domed structure which caused the 
Tonto formation and the formations overlying it to be removed 
from the Algonkian willl be made when the subject of Physi- 
ography is taken up. 
The Upper Silurian. — Immediately overlying the Tonto at 
its outer edge and possibly conformable with it are red to 
brown fossiliferous, coarse-grained lime rocks having a thick- 
ness of about seventy feet. 
They form only a narrow band, but are continuously ex- 
posed from the Tonto basin to Nantan mountains, where not 
covered with talus or lava. The best exposures observed by the 
writer are on the trail to the Salt springs, some ten miles south- 
west of the farmer's residence on Cibicu, and on the John 
Dazen trail near the Oak creek break about a half mile south- 
east of Oak creek cliff houses. This formation is Silurian, at 
least the fossils obtained, Orthis davidsoni, Strombodes penta- 
gonus, etc., seem to bear out this conclusion. 
The Devonian. — Immediately overlying the Silurian on the 
.periphery are alternating chert and flint strata followed by 
massive, very fossiliferous, light colored, fine grained, marble 
limestone, which in turn is followed by a grit. This formation, 
