74 REVIEWS — THE GEOLOGICAL SURVEY OF MICHIGAN. 



the limits of the State. Commencing with the most ancient, we have 

 first a collection of metamorphic beds, comprising talcose and chloritic 

 schists, with quartzites, siliceous slates, and bands of crystalline lime- 

 stone, the whole being undoubtedly referrible to the Laurentian series 

 of our Azoic rocks. The great iron deposits of Marquette belong to 

 this division. The existence of Huronian strata within the peninsula 

 does not appear to have been made out. To these Azoic rocks succeed 

 the so-called " Lake Superior sandstones " of the age of the Potsdam 

 group, but they are only developed in the higher portions of the 

 State, The Calciferous sandrock is said to be unknown to the south 

 of the Falls of Ste. Marie, the silicious and other limestones of the 

 Trenton group immediately succeeding the Potsdam beds. It may be 

 found, however, eventually, that much of the Lake Superior sandstone 

 really belongs to the Calciferous subdivision, although we have not suf- 

 cient data at present to establish this. ]\lany of the common Chazy and 

 Trenton limestone fossils are cited by Professor Winchell from this out- 

 crop of the Trenton group ; and the formation is stated to stretch in a 

 belt about four miles wide across St. Joseph's Island, and to reappear ia 

 the high bluffs opposite Little Sailor Encampment. From thence it ex- 

 tends across the middle of Great Sailor Encampment Island, and passes 

 west in a gradually widening belt to the shores of Bay de Noquet and 

 Green Bay, and onwards across Wisconsin into northern Illinois. 

 Thirty-two feet is cited as its observed thickness in Michigan. The 

 Hudson River deposits lie along the southern outcrop of the Trenton 

 beds. Their dip carries them under the lower portion of the penin- 

 sula, but they do not reappear on the southern side of the basin, 

 being covered at the anticlinal by some of the overlying beds. They 

 are seen, however, further south, as in the denuded axis of Cincinnati, 

 and elsewhere. 



The Upper Silurian formations recognized in Michigan, comprise 

 the Clinton and Niagara group, and the Onondaga-salt division. Some 

 gypsum and a few brine springs occur in the latter, but the great salt 

 formation of the peninsula belongs to a much higher deposit, a member 

 of the Carboniferous series. The Devonian rocks are largely developed. 

 They include, in ascending order, the upper Helderb erg group, 354 feet 

 thick, comprising the peculiar brecciated limestone of Mackinac, and 

 other arenaceous and bituminous limestone deposits ; the Hamilton 

 group of bituminous and other limestones, 55 feet in thickness ; the 

 Portage group, 224 feet thick; and the Chemung group of 159 feet. 



