SUMMARIES OF CURRENT NORTH AMERICAN PRE- 

 CAMBRIAN LITERATURE r 



Clements, Smyth, Bailey, and Van Hise 2 describe the Crystal 

 Falls iron-bearing district of Michigan. 



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

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

 kian 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 applied 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 Huro- 

 nian series which there occur. 



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 Sturgeon quartzite, from ioo feet to more than iooo feet thick; 

 the Randville dolomite, from 500 to 1500 feet thick; the Mansfield 

 slate, from 100 to 1900 feet thick; the Hemlock volcanic formation, 

 from 1000 to 10,000 or more feet thick ; and the Groveland formation, 

 about 500 feet thick. A minimum thickness for the series is about 

 2200 feet, and a possible maximum thickness is more than 16,000 feet. 

 However, in the latter case, a large part of the series is composed, of 

 volcanic material. It is not likely that the sediments at any one place 

 are as much as 5000 feet thick. 



The Upper Huronian is a great slate and schist series, which it is 

 not possible to separate on the maps into individual formations. It is 

 impossible to give even an approximate estimate of the thickness of 

 this series. 



'Continued from page 443, Vol. VIII, Jour. Geol. 



2 The Crystal Falls Iron-bearing District of Michigan, by J. MORGAN CLEMENTS 

 and H. L. Smyth, with a chapter on the Sturgeon River Tongue, by W. S. Bayley, 

 and an Introduction by C. R. Van Hise : Mon. U. S. Geol. Surv., No. XXXVI, 1899. 

 With geological maps. 



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