440 CO. D. Walcott—Pre-Carboniferous Strata in Arizona. 
The great unconformity beneath the Tonto has been 
described by Professor J. W. Powell, who examined it in his 
boat trips down through the Grand Cafion, and by Captain C. 
E. Dutton, who viewed it from the summit of the Kaibab Pla- 
teau, five milesaway. Professor Powell estimated the strata be- 
neath the Tonto and above the Archean at 10,000 feet, and as 
this is all cut across the unconformity is very great. A detailed 
study adds to the thickness of the strata; it shows that the orig- 
inal summit of the Pre-Tonto group had undoubtedly been cut 
away more or less before the deposition of the Tonto group; 
that the plane of erosion cut deeply into the Archean, and that 
besides the 13,000 feet of strata, that have been planed off, 
of which the record is found in the section preserved, there 
character of the sediments. The lower, the Grand Cafion 
group, is made up of an immense mass of sandstones and 
interbedded greenstones exposed directly on the Grand Cafion, 
and the upper, the Chuar group (a name given by Professor 
Powell), is a series of sandy and clay shales in the interbedded 
sandstones and limestones that are exposed in the inner cafion 
valleys between the Kaibab Plateau and the six great buttes 
forming the west side of the lower portion of Marble Cafion. 
The summit of the Chuar group is, as now known, In a little 
synclinal on the divide between Nun-ko-weap and Kwa-gunt 
8. . 
At first a.rough sandstone, it gives way below to sandy and 
thin argillaceous shales with interbedded sandstones and lime- 
stones, 285 feet of limestone occurring in 5,170 feet of sedi- 
ts 
stones resting on a massive belt of greenstones 1,000 to Le 
feet in thickness ; this belt is broken up into eight principa 
ows by partings of sandstone deposited between the flows. 
accumulated on the upturned and eroded edges of the Archean, 
the few layers of limestone and the one flow of lava, 150! 
in thickness near the base, scarcely serving to break the great 
sandstone series. The Archean, where the section terminales, 
consists of thin-bedded quartzites broken by intrusive velns © 
* For position of these valleys see Atl f 5 rtiary History of the 
Grand Cafion, 1882. y as of Dutton’s Tertiary 
