HISTORICAL SKETCH. XX1x 
the same place he mentions Receptaculites over 50 feet below the base of the Galena lime 
stone. The same occurs at Elkader mills. 
The Galena is estimated, at Hlkader mills, as 130 or 140 feet thick, and at Dubuque 
as 200 or 250 feet thick. ‘‘From all the sections measured it is very certain that the Galena 
limestone gradually thins out to the north and northwest, and at the same time loses 
very much the characteristic features which distinguish it in the productive lead region.” 
The uppermost beds at Elkader mills are said to be ‘‘black on fresh fracture, weathering 
to light gray or drab,” an evenly bedded limestone with shaly partings. 
The Hudson River group, which had been described by Percival in Wisconsin under 
the name of ‘‘Blue shale,” was recognized by Hall in Iowa, occupying a slope usually 
without exposure of rock, situated between the Galena limestone and the magnesian 
limestone which forms the capping of numerous mounds in the northeastern part of Iowa. 
These beds were found to be characterized by great numbers of small orthoceratites and 
Nucula. The basal member is a black slate ‘‘not unlike the Utica slate,” and contains two 
species of Lingula, one much larger than the other. The total thickness of these beds is 
not more than 60 feet. ‘‘The term ‘Blue limestone’ was originally applied in the Ohio 
geological reports to the shales and limestones of the Hudson River group as developed 
in the neighborhood of Cincinnati, and these were formerly supposed to be the continuation 
of the Trenton limestone of New York.” The fossils described in Part Il of this report 
are Devonian and Carboniferous. 
J. D. Whitney. 
_ 1858. In the same volume as the last noted the chapter on « Chemistry and Econom- 
ical Geology” is by professor J. D. WHirNEy. He reviews the geological succession in 
lowa. 
The term ‘‘ Blue limestone” here is made the equivalent of the Trenton, including all 
the strata from the St. Peter to the Galena, and the Buff limestone is a subordinate 
member at the bottom, which for convienence of description could be distinguished “from 
the Blue limestone proper.” The Buff varies from 15 feet to 20 feet in thickness, and the 
Blue proper from 70 feet to 80 feet. In the discussion of the stratigraphy and the chemical 
composition it is not plain whether the author speaks, generally, of the Blue limestone or of 
the ‘‘ Blue limestone proper.” The ‘‘glass rock” characters are common near the bottom. 
This is a nearly pure carbonate of lime, fine-grained, imperfectly crystalline, easily 
breaking into cuboidal blocks with a smooth, often conchoidal fracture. The passage from 
the Trenton to the Galena is by a series of alternations of purely calcareous and calcareo- 
magnesian layers. 
The greatest thickness of the Galena is at Dubuque, 250 feet, and from that point it 
seems to thin out in all directions. Thisis a dolomyte, and resembles the Lower Magne- 
ian limestone. Toward the top it becomes shaly, and gradually passes into the Hudson 
River shales. The central portion is massive, coarsely crystalline, with cavities that 
appear on weathering, with chert and other siliceous impurities, and the lower beds become 
