172 Th( American Geologist. September, 1894 
limestone in Iowa, (2) by the dissolving of some portion of 
the stratum subsequent to the formation, lithifiieation, and 
elevation of the rocky strata above sea level, brecciation of 
the remainder, and reeementation,by percolating waters, as in 
the dolomites of the Ozark series in Missouri, and (3) by the 
breaking up of a stratum by the waves on a rocky sea shore 
previous to the deposition of the overlying formations, as in 
the Lower Magnesian limestone in Wisconsin, the last is 
considered (after having studied the matter as closely as pos- 
sible with the very limited outcrop) as the only process by 
which the breccia under discussion could have been produced. 
The two heavy-bedded dolomitic strata, numbers 3 and 5, 
resemble the Galena limestone and still more strongly the Buff 
limestone underlying the Trenton proper. But they are more 
compact, finer grained, and lighter colored than the Galena, 
and are a purer dolomite than the Butt', which always contains 
considerable argillaceous material. Furthermore, there is a 
tendency to develop exfoliation or something similar to it, by 
No. 5, on erosion. The very thin-bedded silicious dolomite, 
No. 4 of the section, is characteristic of this formation, no- 
where appearing in any of the formations above or below. 
Enclosed between two heavy-bedded pure dolomite strata, its 
white, cherty, one inch layers are in strong contrast to them. 
There are also thin layers of pure sand. 
There are indications of fossils, mostly the almost obliter- 
ated casts of gastropods, in the dolomite. One imperfect 
specimen was secured, but the formation may be considered 
as onl}* slightly fossiliferous. 
With what formation of neighboring states may this be 
correlated? Of the few wells in the immediate vicinity, one 
at least penetrates the limestone, hut not to any considerable 
depth. So the only means of determining its thickness is the 
outcrop, which has a vertical extent of 10 feet. It is very 
probable, however, that it extends down many feet below the 
creek level. From the outcrops and well sections of the St. 
Peter sandstone in Illinois and neighboring portions of Wis- 
consin, no intercalated stratum of limestone or shale has ever 
been reported. Nor does it at all seem probable that here in 
this one small spot in the early Silurian sea, dolomite and 
shale would be deposited, while sandstone was being laid 
