IOWA ACADEMY OF SCIENCES. 
41 
impure limestone that breaks up on exposure to the weather 
into many small angular fragments. These are the beds of 
passage, and they are followed by heavy-bedded dolomite of the 
Niagara age. 
At Colesburg the well at the hotel was drilled through 60 feet 
of loess, 10 feet of flint and red clay, which evidently embraces 
the residual cherts and clays of the geest; and through 183 feet 
of Niagara limestone and beds of passage to the blue shales of 
the Maquoketa age. Water was found at the top of the shale. 
The di]3 at Colesburg is normal. The synclinal axis already 
referred to evidently lies a little to the west. 
About six miles southwest of Colesburg, in the southern 
edge of section 2, T. 90 N. , R. IV W. , the valley of Elk creek, 
has been eroded down into the shale and furnishes some fine 
exposures. As before the shale is non-fossiliferous, and admir- 
ably adapted to the manufacture of pottery. Exposures occur 
at intervals for a mile or two up and down the creek from the 
point described above. 
At a number of points in the valley northeast of Greeley the 
beds of passage which lie immediately above the blue shales of 
the Maquoketa, are exposed and exhibit their usual character- 
istics. 
The shales themselves are not seen. The best example of 
the beds of passage is seen at the mill at Rockville. At this 
point they were found to contain a few fossils belonging to Hud- 
son river types, which shows that the lower part, at least, of 
these beds must be referred to the Maquoketa rather than the 
Niagara age. Lithologically they resemble some portions of 
the Niagara, but the contained fauna is Ordovicia or Lower 
Silurian. 
The blue argillaceous shales are not naturally exposed at 
Rockville, but the water pouring over the dam has excavated a 
deep pit as the result of the plunge, scooping out great masses 
of the shaley portions of the beds and piling them up in heaps 
and ridges a short distance below. These heaps contain many 
beautiful fossil-bearing shales, and in the multitude of finely 
preserved individuals we recognize Orthis testudinaria, Plectam- 
bonites sericea, Stropliomena JUitexta, StropUomena rugosa, Zygos- 
gyira modesta and Calymene senaria. In the varietal characters, 
mode of preservation and association of the species, the slabs 
from Rockville could not be distinguished from specimens of 
corresponding character collected at Cincinnati, Ohio. 
