REVIEWS — THE GEOLOGICAL SURVEY OF MICHIGAN. 75 



The Portage group in this Report, is called provisionally the " Huron 

 group," an objectionable term, as likely to be confounded with the 

 very dissimilar Huronian series of Canada. The Chemung subdivi- 

 sion is also described as the Marshall group, alter the town of that 

 name in Calhoun county. In neither of these sets of strata have 

 any explorations been made, as yet, in quest of petroleum ; nor have 

 any natural "oil springs" been discovered amongst them. The 

 fossils collected from the higher group are stated by Professor Wiu- 

 chell to be specifically distinct from those obtained from the Chemung 

 beds of more eastern localities. 



Continuing to ascend in the geological scale, we now reach the great 

 Carboniferous formation. This, as occurring in Michigan, is subdivi- 

 ded in the Report into six separate groups, according to the following 

 order. First, a series of micaceous sandstones, the so-called "Napo- 

 leon group," separated from the underlying Marshall deposits by a 

 bed of clay or argillaceous shale. Secondly, the " Michigan salt 

 group," comprising various shales and limestones, with gypsum and 

 marl beds. This is the great brine formation of Michigan, a kind 

 of repetition, as it were, of the Onondaga salt or gypsiferous group of 

 more eastern sites. Some of the sandstones of the lower or " Napo- 

 leon" group, however, are also highly saliferous. Next follows the Car- 

 boniferous Limestone, 66 feet thick, succeeded by the so-called Parma 

 Sandstone, a subdivision considered to represent the Ohio conglomer- 

 ate, though here separated from the Devonian rocks by three inter- 

 vening groups of calcareous and arenaceous deposits. The Coal 

 Measures come next in order, attaining in some places to a thickness 

 of over 120 feet. The coal seams, which consist of bituminous 

 varieties, are said to reach in the aggregate a thickness of eleven feet. 

 A single seam, averaging from three to five feet, appears to extend 

 continuously throughout the formation, and to furnish material of 

 good quality. It is chiefly worked at Woodville. Fire clay and some 

 thin bands and nodules of iron-stone occur in connexion with it. 

 Above the Coal Measures, a thick arenaceous deposit is met with. This, 

 which much resembles the rock beneath the coal strata, is called the 

 Woodville Sandstone ; but Professor Winchell remarks that these 

 three latter formations, though separated for convenience, should 

 strictly be united, and ranked collectively as the Coal Measures 

 proper. With the Woodville Sandstone, the entire Palaeozoic series 

 of Michigan is brought to a close ; and the next succeeding deposits 



