196 The American Geologist, \pril, 1896 
miles north of Postville. [minediately north of the town, how- 
ever, the river has not cut quite through the Trenton, but by 
following down the channel the formations lying beneath the 
Trenton are seen in orderly succession. 
At Postville, and in the valley of the Yellow river north of 
the town, the Trenton limestone attains an unusual thickness. 
Between the Bull' beds, that are undoubtedly Galena, anil the 
top of the Saint Peter sandstone there occurs tin- following 
section, the measurements being closely approximate but not 
exact. 
Feel. 
8. Blue clay, unfossiliferous 5 
7. Thin bedded argillaceous limestone, no fossils .") 
('). Bluish shales with some fossils .") 
5. Thin-bedded, gray, shaly limestone, fossiliferous CO 
4. Blue, fine-grained limestone, weathering to buff, with 
many thin beds of shale near the top and frequent 
beds of yellowish, argillaceous limestone 200 
3. "Green shales," with many thin layers of very fossilifer- 
ous limestone: containing Orthis subcoquata, O. tri- 
cenaria, O. bellarugosa, Strophomena filiiexta and 
other brachiopods in the lower part, and many in- 
dividuals of Prasopora simulatrix and related bry- 
ozoans in the upper part 40 
2. Heavy bedded, magnesian limestone. Lower Buff beds of 
the Wisconsin geologists 25 
1. Basal shale resting on Saint Peter sandstone 5 
Total 345 
Number 4 of this section contains the "Blue limestone 11 that 
is generally counted as the typical portion of the Trenton. 
The Minnesota geologists, however, have sometimes regarded 
the ; 'green shales" as the upper limit of the Trenton, and have 
referred all Lower Silurian sediments above No. 3, of the pre- 
ceding section to the Galena limestone and Hudson River, or 
Maquoketa shales.* At Postville, Iowa, the Galena proper is 
represented by bull', dolomitic beds that overlie all the lime- 
stones and shales exposed in the section described. South 
of Postville the Galena passes beneath the true Maquoketa. or 
Hudson Kiver beds, while a little farther south, as for exam- 
ple at the Williams 1 quarry, tin 1 shales in turn disappear be- 
neath outliers of the Niagara. It would seem, therefore, not- 
withstanding their extraordinary thickness, that the whole 
aggregate of sediments included in numbers 1 to S of the sec- 
tion must be referred to the lower division of the Trenton 
*Geology of Minu., vol. i of the Final Report, p. 293. 18S4. 
