102 THE GEOLOGY OF MINNESOTA. 



.' ^ J^'l I. {Hurlbut, 1871. 



. .> V; ,. . . 



HURLBUT ON THE GEOLOGY OF SOUTHERN MINNESOTA. 



In 1871 Mr. W. D. Hurlbut, of Rochester, Minnesota, contributed a series 

 of papers to the Minnesota Teacher on the geology of southern Minnesota, 

 which were subsequently issued together as a pamphlet. These papers 

 supply a lack, which was a conspicuous and remarkable one, in the geologi- 

 cal literature of the state considering the general accuracy and fullness of 

 Owen's report since no geologist had before penetrated this part of Min- 

 nesota, and nobody had called attention to its marked topography or to its 

 geology. Owen's parties passed around it. They ascended the Mississippi, 

 the Minnesota and the Des Moines, but the valleys of the Root and the 

 Zumbro were not examined. It is in these valleys, and particularly on the 

 upper tributaries, that the upper parts of the Silurian and the Devonian 

 are found exposed. 



Taking the Mississippi river, and the measurements aud descriptions ot 

 Dr. Owen, as initial points, Mr. Hurlbut follows up the streams coming from 

 the west, across the strike of the formations, noting the changes as they 

 occur in the strata, and stating their main characteristics and thicknesses. 

 He thus makes out the Potsdam, the Lower Magnesian, St. Peter sandstone, 

 Trenton limestone flags, Hudson River shales, argillaceous shales which he 

 regards of the age of the Clinton, and the Devonian. He also outlines their 

 geographical extent, and states some of their topographic features. His 

 identifications, being the first recorded attempt to parallelize those strata 

 with any recognized base of nomenclature in the state of Minnesota, and 

 dependent for the greater part on lithological features, were subject to such 

 changes as a study of the fossils might require. His Hudson River shales 

 were restricted to the very base of the rocks of that formation, and desig- 

 nated " Hudson River oil shales," having a maximum thickness of fifteen feet. 

 They are the " Green shales" of the early reports of progress of the survey, 

 and probably belong to the Hudson River group. His shaly limestone (Clin- 

 ton) is the upper part of the Hudson River, becoming in some places a very 

 calcareous member almost without shales. His Devonian, in which the 

 arenaceous parts were supposed to be Schoharie sandstone, is the buff mag- 

 nesian limestone of the Galena. The elevated land, further southwest from 

 the strike of the last, in Mower and Fillmore counties, he suggests may con- 



