Upper Silnridit ill JS'orfheftsfeni lova. — Wilson. 211 
members, though it is stated in the text tliat this may be only 
false bedding. In this report Keyes says,* "The exact sub- 
divisions of the Upper Silurian rocks in Iowa are yet some- 
what unsettled. * * * With the exception of White, 
[McGee's memoir had not yet appeared] all geologists who 
have examined the Upper Silurian strata in Iowa regard the 
rocks as made up of at least two distinct formations. At 
present these subdivisions differ greatly, not only faunally, but 
in a less marked degree in their stratigraphical and litholog- 
ical characters. For reasons set forth above, Hall's Le Claire 
i=!eems a desirable name for the upper member as now under- 
stood, while Niagara, for the present, will be retained for the 
lower." 
In the report last quoted there is also an article by G. L. 
Houser on "Building Stones and Limeburning Dolomites of 
the Niagara in lowa.^f The subdivisions recognized in this 
are the upper Niagara, affording the building stone, and the 
middle Niagara, said to be the best for lime ; thus impl^nng 
that there is a third or lower member. An article by J. P. 
Farnsworth in the American Geologist for Nov., 1888, per- 
taining to the Niagara in Iowa does not mention subdivisions 
of the group, neither do two articles by Calvin on fossils of 
the Niagara in the American Geologist for July and August, 
1893. 
The object of this paper is to state the result of a study of 
these rocks which has extended over several years, with an 
attempt to describe the subdivisions as they appear in count- 
less outcrops in Delaware. Jones, Dubuque and Chiyton 
counties. 
The basal member of the formation, as determined by out- 
crops of the Maquoketa shales beneath, is well exposed at the 
following places : (1) at Hockville, Delaware C'o. ;;J; ('!) two 
miles northeast of Colesburg; (3) on a branch of Elk creek, 
five miles northeast of Greeley; (4) in the Mississi]i])i l)luffs 
near Sabula. 
This l)asal portion, for whicli Calvin has used the term 
"•beds of ])assage," is comi)osed of thin bedded, non-vesicular, 
l)utl' colored dolomite, the layers being from one to three inches 
*P. .30. ~ 
tOp. cit., p. 20.3. 
tSee article by Calvin. ProL-eodinj^'s Iowa At-ad. Sci.. 1.S94. j). 40. 
