II^TRODUCTIO]^. 



By C. R. Van Hise 



This report is a full account of the Crystal Falls iron-bearing district 

 of Michigan. 



The rocks of the district comprise two grou2JS, separated by uncon- 

 formities. These are the Archean and the Algonkian. The Algonkian 

 includes both the Lower Huronian and the Upper Huronian series, and 

 these are also separated by unconformities. The terms Lower Huronian 

 and Upper Huronian are apj)lied to the series which occur in this district 

 because they are believed to belong to the same geological province as the 

 Huronian rocks of the north shore of Lake Huron, and to be equivalent to 

 the Lower Huronian and Upper Huronian series which there occur. The 

 reasons for this belief are fully given in Bulletin 86.^ 



The Archean is believed to be wholly an igneous group, and there- 

 fore no estimate of its thickness can be given. It covers a broad area in 

 the eastern part of the district, and from this several arms project west. 

 West of the main area there are two large oval areas of Archean. 



The Lower Huronian series, from the base upward, comprises the Stur- 

 geon quartzite, from 100 feet to more than 1,000 feet thick; the Randville 

 dolomite, from 500 feet to 1,500 feet thick; the Mansfield slate, from 100 

 feet to 1,900 feet thick; the Hemlock volcanic formation, from 1,000 feet to 

 10,000 or more feet thick; and the Groveland formation, about 500 feet 

 thick. We thus have a minimum thickness for the series of about 2,200 

 feet, and a possible maximum thickness of more than 16,000 feet. However, 



' Correlation papers, Archeau and Algonkian, by C. K. Van Hise : Bull. U. S. Geol. Survey No. 86, 

 1892, pp. 156-199. 



MON XXXVI II xvii 



