HISTOKICAL SKETCH. XXXV 



when heated, but which show distinct impressions of a vegetable character. * That the same 



fossils which are characteristic of the Galena limestone in the lead region are found in the Blue, beyond 

 the limits of our district, to the northwest, is a fact observed during the progress of the Iowa survey. It is 

 evident that after crossing the Mississippi, and proceeding beyond Gutenberg in that direction, the 

 Galena and the Blue limestones become more and more merged in each other, and less distinguishable 

 either by paleontological or lithological characters. " 



In plate IV, giving a section of the rocks exposed in the lead region, the Galena and 

 the Blue are together said to be the equivalent of the Trenton limestone of New York. 



Mr. Whitney retains the term Hudson River group for the next overlying formation 

 the Green and Blue shales and limestones of Prof. Hall. It is given a thickness, in the 

 lead region, from 70 to 100 feet. It is shaly, but holds some beds of dolomyte. The shale 

 is sometimes carbonaceous in sufficient degree, perhaps, to make it, in the future, of 

 economical value, and is marked by traces of graptolites. 



James Hall. 



1863. Note on the geological range of the genus ReceptaculHes in American Paleozoic 

 strata. JAMES HALL. Sixteenth report of the New York State Cabinet, pp. 67-70, 1863. 



Five species have been described from the Galena limestone, viz. , oweni, iowensis, 

 fungosus and obicularis, and one from the Trenton limestone, occidentalis (neptuni?), in 

 New York. 



F. B. Meek and A. H. Worthen. 



1868. Geological Survey of Illinois, Vol. Ill, Paleontology, P. B. MEEK and A. H. 

 WORTHEN, Springfield, 1868. 



This volume embraces descriptions and figures of fossils from the Trenton, Galena 

 and Hudson River formations, viz., from the Trenton one echinoderm, three lamelli- 

 branchs, one cephalopod and one crustacean; from the Galena, one pteropod, one zoo- 

 phyte, one brachiopod, five lamellibranchs, five gasteropods, one cephalopod, and two 

 crustaceans; from the Hudson River, six echinoderms, three brachiopods, one gasteropod 

 and three pteropods. 



C. A. White. 



1870. Report of the Geological Survey of Iowa. CHARLES A. WHITE. Vol. I, Des 

 Moines, 1870. On pages 174-182 the Trenton group, of the Lower Silurian, is treated. 

 The Trenton limestone proper is made to include the strata between the St. Peter sand- 

 stone and the Galena limestone. Along the bluffs of the Mississippi its thickness is about 

 eighty feet, but in Winnishiek county it increases to above two hundred feet. 



The Galena diminishes in thickness from Dubuque northwesterly, from 250 feet to 

 probably 100 feet on the northern state boundary, where it also has a greater westerly dip. 



The name Maquoketa shales is given to the shales lying above the Galena limestone 

 and separating it from the Niagara limestone. They are said to be characterized by 

 peculiar faunal features, ( "Orthoceras, Murchisonia, Pleurotomaria, Schizodus(?), Dis- 

 cina, Graptolithus, etc.,") sufficient to warrant their assignment to a very low horizon in 

 the Cincinnati group. 



