176 REVIEWS 



They have been assigned to the Archean because of lithological characteristics, but the 

 contact between them and the Lower Huronian sediments to the north is covered by 

 Upper Huronian and glacial deposits. Greenstones to the west on the Brule River, 

 similar to the Quinnesec schists, are closely infolded, and perhaps interbedded with 

 slates which have been mapped as Upper Huronian. In view of these facts, it is 

 quite possible that the Quinnesec schists are as late as Upper Huronian, thus corre- 

 sponding to the Clarksburg volcanics of the Marquette district. 



On the basis of the triple division of the Huronian series which has been adopted 

 since the discovery of an unconformity in the previously called Lower Huronian series 

 of the Marquette district, the Upper and Lower Huronian series are represented in the 

 Menominee district, and not the Middle Huronian, unless we except certain pebbles 

 taken to represent the Negaunee formation. It is still possible that the iron formation 

 mapped as Upper Huronian may in reality be Middle Huronian, as suggested by partly 

 hypothetical structural connection with formations mapped as Middle Huronian to 

 the north, but in this case there should be found an unconformity between the iron 

 formation and the slates above, and none has yet been found, although the field has 

 been most carefully examined. 



S. Weidman. "The Baraboo Iron-Bearing District of Wisconsin." Bulletin 

 XIII, Wisconsin Geological and Natural History Survey, 1904. Pp. 190, 

 with map. 



Weidman describes and maps the Baraboo quartzite region of south- central 

 Wisconsin. 



A pre-Cambrian quartzite formation, having an estimated thickness of 3,000- 

 5,000 feet, forms an east-and-west synclinorium about 20 miles long, and ranging in 

 width from 2 miles on the east to 10 or 12 miles on the west, resting on a basement of 

 igneous rock consisting of granite, rhyolite, and diorite, in isolated and widely sepa- 

 rated areas both north and south of the quartzite synclinorium. The largest area is 

 one of rhyolite near the lower narrows of the Baraboo. The upturned north and 

 south edges of the quartzite form respectively the North and South Ranges of the 

 Baraboo Bluffs, standing 700-800 feet above the surrounding country and above the 

 intervening valley. In the valley are pre-Cambrian formations younger than, and 

 conformable with, the quartzite. These are the Seeley slate, having an estimated 

 thickness of 500-800 feet, and above this the Freedom formation, mainly dolomite, 

 having a thickness estimated to be at least 800 feet, bearing iron-ore deposits in its 

 lower horizon. 



Flat-lying Paleozoic sediments, unconformably overlying the pre-Cambrian rocks, 

 occupy the surrounding area and partly fill the valley. The Paleozoic rocks range 

 from Upper Cambrian, Potsdam, in the valley bottom to the Lower Silurian, Trenton, 

 on the upper portions of the quartzite ranges. The Potsdam sandstone has a thick- 

 ness ranging from a few feet to a maximum of about 570 feet in the valley. Glacial 

 drift is abundant over the quartzite ranges and in the valley in the eastern half of the 

 district, but occurs only in the valleys in the western half. 



The iron ore is mainly a Bessemer hematite with soft and earthy, hard and black, 

 and banded siliceous phases. A very small amount of hydrated hematite or limonite 

 is also present. The rocks immediately associated with the ore and into which the 

 ore grades are dolomite, cherty ferruginous dolomite, ferruginous chert, ferruginous 



