HISTORICAL SKETCH. XXIX 



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 Elkader mills, as 130 or 140 feet thick, and at Dubuque 

 as 200 or 250 feet thick. "Prom 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 II 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. WHITNEY. He reviews the geological succession in 

 Iowa. 



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 Buffi 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. This is 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 



