100 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
The relations of the Grand Caiion section are shown in the first 
column of the page of sections. 
The Potsdam sandstone in Wisconsin occupies the same relative 
stratigraphic position as the Tonto formation, except that the break 
above the Tonto and between it and the Devonian is filled in by 
the Calciferous and other Silurian formations. As has already 
been said, the faunas of the Potsdam and Tonto are very much the 
same in general character. The Potsdam formation here overlies 
unconformably a series of strata that are directly comparable with 
the Grand Cafion and Chu-ar’ series. The Keweenawan series, 
according to Chamberlin, has about 10,000 feet of sedimentary 
strata distributed through 30,000 feet of eruptive rocks. In all 
this great mass no decisive evidence of organic life has been dis- 
covered, but knowing that the series is unconformably overlain by 
the Potsdam formation and that it in turn rests unconformably on 
the Archean, as does the Grand Cajion series, we feel justified in 
correlating the Grand Cajion and Wisconsin sections and they are 
united in the first column of the page of sections. 
The upper part of the Nevada section has already been men- 
tioned. Below the Potsdam horizon there occurs a distinct fauna, 
characterized by a considerable development of the trilobitic genus 
Olenellus, a genus that in the embryonic development of several of 
its species proves that it is derived from the Paradowides family 
and is consequently of later date. This section is readily correlated 
with that of the Georgian group of Vermont, as there we have the 
Potsdam sandstone above the Olenellus horizon, and in the down- 
ward section both stop at nearly the same relative horizon. The 
position of the Georgian formation in Nevada and Vermont, in 
relation to the Potsdam, leads to the view that it represents a por- 
tion of the period of erosion between the Tonto formation and the 
Grand Cafion series and also the Potsdam formation and the 
Keweenawan series. 
The upper portion of the Tennessee Cambrian, the Knox shale, 
is correlated with the Potsdam sandstone, and so is the Knox sand- 
stone. The Chilhowee sandstone and Ocoee conglomerate and 
slates cannot be directly connected with the Georgian horizon, 
since the paleontologic data are insufficient. From their position 
beneath the Knox shale with its Potsdam fauna they are extended 
downward past the Georgian and into the Paradoxidian or St. Johns 
horizon. Their total thickness (Geology of Tennessee, pp. 158, 
159) is nearly 15,000 feet. 
