XXVI INTEODUCTION. 



cession in the Marquette, Crystal Falls, and Menominee districts is substan- 

 tially the same. The Clarsltsburg formation in the Marquette district may 

 be omitted from consideration, because it is volcanic and replaces in part 

 the Michigamme and Ishpeming formations. The Upper Huronian was a 

 great period of slate and grit deposition. The chief difference which appears 

 between the Menominee district and the other two districts is that in the 

 former no iron ores have been found in the Upper Huronian within the 

 district proper, although such rocks occur a short distance to the west, at 

 Commonwealth and Florence, in Wisconsin. 



The succession for the Lower Hm'onian in the tliree districts can be 

 liaralleled with a high degree of probability. The chief diflFerences are 

 due to the disturbance of the great volcanic outburst in the western part 

 of the Crystal Falls district and to the uneven surface of the Archean land 

 at the beginning of Lower Huronian time. As a consequence of the latter, 

 the waters did not reach the western part of the Marquette district and the 

 northeastern part of the Crystal Falls district as early as the eastern part of 

 the Marqtiette district, the central part of the Crystal Falls district, and the 

 Menominee district. The transgression of the Lower Huronian sea for the 

 region covered in these three districts was therefore from the southeast 

 toward the northwest. 



The Negaunee iron formation of the Marquette disti'ict is equivalent to 

 the Grroveland iron formation of the Crystal Falls disti'ict and the Vulcan 

 iron formation of the Menominee district. 



The Siamo slate and the Ajibik quartzite of the Marquette district are 

 approximately equivalent to the Hemlock volcanic formation in much of 

 the Crystal Falls district, but in places where the latter formation displaces 

 the Mansfield formation they are equivalent to only a part of the Hemlock 

 volcanic formation. The Wewe slate of the Marquette district is equivalent 

 in the western part of the Crystal Falls disti-ict to a part of the Hemlock 

 volcanic formation, and in the southeastern part of the district is probably 

 equivalent to a part of the Randville dolomite. It appears that the Siamo 

 slate, Ajibik quartzite, and Wewe slate of the Marquette district, and the 

 Mansfield and Hemlock formation of the Crystal Falls district, are equiv- 

 alent to a part of the Antoine dolomite of the Menominee district. 



The great dolomite formation occurring in all of the districts is sup- 

 posed to be equivalent, except that, as just explained, the deposition of 



