328 PEL Ji OOK NAL OP NG OEO Give 
Nevada. The stratigraphic position of the sandstone at the base 
of the Tonto terrane is that of the Middle Cambrian, a-horizon 
equivalent to that of the lower portion of the St. Croix sandstone 
of Wisconsin and the Secret Canyon shale of the Eureka District 
of Nevada. The fauna of the Chuar terrane indicates the pres- 
ence of life, but it is not of value in stratigraphic correlations. 
It is probable, almost to a certainty, that it is older than the 
Olenellus fauna of Nevada. If this be true, the interval between 
the summit beds of the Chuar terrane and the Tonto sandstone 
is represented, in Nevada and Utah, by a deposition of 3000 or 
more feet of limestones and many thousand feet of sandstones 
and siliceous argillites. With the exception of a few traces at 
the base of the Tonto sandstone, none of detrital sediments 
resulting from the erosion of the pre-Tonto land area have been 
discovered. 
GEOLOGIC AGE. 
The lower portion of the Tonto terrane, the Tonto sandstone, 
forms the base of the Paleozoic section in the Grand Canyon 
District. It is massive-bedded and rather coarse in the lower 
portion, passing above into shaly, fine-grained, fossiliferous sand- 
stones. The presence of a well-marked Middle Cambrian fauna 
in its upper portion clearly indicates its geologic age. It is only 
the absence at the base of the sandstone of the Lower Cambrian 
or Olenellus fauna that prevents us from carrying the recognized | 
Palzozoic section down to include its oldest known fauna. The 
period of erosion represented by the unconformity between the 
Tonto sandstone and the Grand Canyon series is considered to 
more than equal Lower Cambrian time, and to constitute a well- 
defined boundary between the Paleozoic and pre-Paleeozoic for- 
mations. In my earlier work, in 1883, I referred the Grand 
Canyon and Chuar strata to the Cambrian;* but upon further 
study of the Cambrian rocks and their contained faunas, and in 
view of the extent of the time-break indicated by the noncon- 
Am. Jour. Sci., Vol. XXVI., 1883, p. 441. 
