Ulrich—Paleozoic Systems in Wisconsin. 
101 
more than one sandstone but also that the two dolomite formations 
are not by any means always separated by a sandstone. Moreover, 
it seems that the sandstone beds are in the form of geographically 
limited lenses lying at varying horizons within either of the dolo- 
mitic formations and passing laterally into sand-free dolomites. 
It has been established also that even the Shakopee and the Oneota 
are not coextensive. In some places the Oneota is absent, as at 
Jordan, Minn., and probably also at Eipon and Butte des Morts, in 
eastern Wisconsin. At many other places in Wisconsin the Shak¬ 
opee is absent. In the former cases the absence of the Oneota is 
most probably due mainly to nondeposition. In the latter instances 
we can not be sure that the absence of the Shakopee is not caused en¬ 
tirely by the unusual activity of surface eroding agencies during 
and preceding deposition of the St. Peter sandstone. 
Where both of the dolomitic formations are present and clearly 
exposed it has usually been found possible to point out precisely 
the plane of contact. But the contact zone is a very likely one to 
be affected and correspondingly obscured by secondary dolomitiza- 
tion. In such unfavorable places the boundary was located only 
approximately though with more time at our disposal than was 
available better results might well be expected. In yet other places 
the boundary proved to be quite irregular and was further empha¬ 
sized by mineralization where the hollows contained original car¬ 
bonaceous mud deposits and rather large pebbles of chert. 
Good exposures of the contact were observed in the bluffs and 
quarries at Stillwater, Minn. Here it lies about 60 feet above the 
base of the Oneota which rests unconformably on the Jordan 
sandstone. Above the top of the Oneota the Stillwater section 
shows about 50 feet of Shakopee with usual characters. There is 
no sandstone worth mentioning between the two formations. 
That the surface of the Oneota was eroded here before the Shak¬ 
opee was laid on it is indicated clearly enough by (1) the rela¬ 
tively slight thickness of the lower formation, (2) the absence of 
the fossiliferous cherty zone that is commonly present in the upper 
part of Oneota sections at and to the south of Trempealeau, (3) the 
unevenness of the contact plane which shows irregularities of con¬ 
tour of a foot or more and corresponding dissection across sedi¬ 
mentary planes in distances of less than 10 feet; and (4) the pres¬ 
ence of one to three inches of conglomerate with limestone and 
chert pebbles in a matrix of coarse quartz sand and grains of glau¬ 
conite. 
