CHAPTER III. 



GENERAL SKETCH OF THE GEOLOGY OF MICHIGAN, AND ITS CONNECTION 

 WITH SURROUNDING DISTRICTS. 



The rocks which constitute the solid crust of our earth may- 

 be arranged into great groups according to the following plan: 

 Stratified. 



Fossiliferous. 



Azoic, or unfossiliferous. 

 Unstratified. 



Volcanic, as lava, trap, &c. 



Plutonic, or Granitic, as granite, syenite, &c. 



Geologically speaking, the Fossiliferous strata are higher 

 than the Azoic, while the place of the Plutonic is generally be- 

 low the Azoic; and the relative antiquity of these three classes 

 of rocks is represented by this order of superposition. The 

 volcanic rocks have burst up through the other rocks at various 

 periods, and the same is to some extent true of the Plutonic — 

 some new granites appearing to have been formed since the 

 granitic substratum of the Azoic rocks was formed. The Upper 

 Peninsula furnishes us with abundant examples of all these 

 classes of rocks. After devoting a few words to the unstrati- 

 fied rocks, we shall proceed to speak of the stratified, as nearly 

 as possible, in chronological order, beginning with the oldest. 



I.— PLUTONIC GROUP. 



A belt of granitic rocks comes down from the northwest into 

 northern Wisconsin, and encroaches a few miles over the Mich- 

 igan boundary line between Montreal river and Lac Vieux 

 Desert. At the surface this is separated by a belt of Azoic 

 rocks from another mass of granite, which is probably a contin- 

 uation of the first, and which begins near the head waters of the 

 Sturgeon river, and extends east, gradually widening, until it 

 occupies the region a few miles back from the lake coast, all 

 the way from the Huron river to Presque Isle, at which two 



