44 THE MAEQUETTE IROX BEAIUNG DISTRICT. 



literature of the suV)ject. The ores are divided into the magnetites, red 

 hematites, and browu hematites, all of Avhich seem to be regarded as 

 sedimentary in manner of deposition, but as volcanic in origin. Even the 

 conglomerates of the region are explained as volcanic : 



MucU of the specular ore coutains fragments of angular jasper hi the shape of 

 breccia, evidently the disintegrated portions of trappeau rocks which were precipitated 

 with the ores when the molten mass was thrown into the surrounding waters, proving 

 that these accumulations of ore-beds aud intercalated schist owe their origin to local 

 causes, or that they are not the results of distant formations, but that they are true 

 beds formed by the flow of molten lava, highly impregnated with iron, into the waters 

 that existed around and perhaps over the volcanic vents. (P. 548.) 



Crednek, Hermann. Die Gliederuug der eozoischen (vorsilurischeu) Forma- 

 tionsgruppe Xord-Amerikas. Zeits. gesammt. Naturw., Giebel, Vol. XXXII, ISGS, 

 pages 353-405. 



Hermann Credner, during his visit to North America, made a rapid 

 examination of the pre-Silurian rocks of Michigan and Minnesota, and 

 announced his conclusions regarding them in two articles, of which the 

 first deals with the general relations of the pre-Silurian formations to one 

 another and to the younger rock series for all the explored parts of the 

 United States aud of Canada. 



The author agrees with Kimball in dividing the pre-Cambrian rocks 

 of the Marquette district into two divisions. The basal rocks all over upper 

 Michigan he declares to consist of a series of gneisses, mica-schists and horn- 

 blende-schists, granites, and syenites, which are included together as the 

 Laurentian series. Above these unconformably are the Huronian beds. 

 The principal rocks of the Laurentian are mica-schists. These are inter- 

 bedded with granites, syenites, hornblende-schists, aud gneisses, the whole 

 forming a conformable series 20,000 feet in thickness. Details are given and 

 localities are described in which the relations between the Laurentian 

 and the overlying Huronian series were made out, but these localities are 

 without the limits of the Marquette area, so they do not directly concern 

 us at the present time. It may be mentioned, however, that at Sturgeon 

 River, in the Menominee district, the author observed a great layer of 



