Review of Hall and Whitney's Geology of Iowa. 107 
Name of Group. Lithological charact 
Galena Limestone, Dolomite, ‘ ‘ 250-300 feet, 
: Impure silicious shells an . 
Haoo River Group, { thin limestone bands, Op-198 
Te Sine - Dolomite, 250-800 “ 
imestone, not recognized ex- P 
cept on the Mississippi river, { en ' 
ondag Dolomite, a few thin outliers, only recog- 
* a Ralf Gronp, ; nized on the Miss, river. 
Tn general there is shown a thinning of the members above 
the calciferous sandstone as compared with the series in New 
York, and a disappearance of some of them. A few particulars 
with regard to the various groups are here added. 
_ Potsdam Sandstone.—The very great thickness of this forma- 
tion, which has been shown to exist in the Lake Superior re- 
gion, is limited to the vicinity of the trappean rocks. Proceeding 
southwesterly from the copper-bearing range, we soon find the 
conglomerate to have disappeared, and have no evidence in Iowa 
and southern Wisconsin of the existence of more than 400 
eet at any one point, while the mean development is prob- 
ably not over 250 or 300 feet. The exposures of this rock 
are very limited in Iowa, but it covers considerable surface in 
Minnesota, and still more in central Wisconsin. Bands or in- 
ated masses of conglomerate are almost entirely wanting in 
the sandstone; these, as well as the lines of oblique Jamination, 
appear to be confined to the vicinity of the igneous rocks. There 
18 no member of. the series more persistent, both in lithological 
and paleontological characters in the lower sandstone; it has 
from lon. 73° to lon. 104°, exhibiting everywhere 
the same granular silicious character and characterized’ by the 
Same organic forms, and we have, up to this time, no evidence of 
the existence of organic remains below this formation. 
_ Lower Magnesian Limestone.—This is a mass of dolomite, hay- 
ing a thickness of from 225 to 250 feet, about 200 of which are 
nearly pure, crystalline dolomite, containing from one to ten per 
cent. of silicious sand, mechanically intermixed; the remaining 
f feet are beds of passage into the sandstone below, consist- 
‘ng of mingled and alternating sandstone and dolomite. Fossils 
are extremely rare in this member of the series; a few have been 
observed in Wisconsin in a very imperfect state of preservation, 
but none in Iowa. 
‘pper or St. Peter’s Sandstone.—This repetition of the sand- 
stone underlying the lower magnesian, is also remarkable for its 
Persistence in lithological character and thickness over a great 
€xtent of surface. From La Salle, in Illinois, where it makes its 
Sppearance in a low axis of elevation, underlying the coal-meas- 
ures unconformably, to St. Paul in Minnesota, a distance of over 
Ped miles, this sandstone hardly varies more than ten feet from 
normal thickness of about 80 feet, which indicates a remark- 
