224 C. D. WALCOTT PRE-CAMBRIAN FOSSILIFEROUS FORMATIONS 



horizons in the slates, and also banded lean magnetic iron ores similar 

 to those of the Penokee region of Wisconsin. There are also interbedded 

 eruptives, consisting mainly of olivine-gabl)ros and orthoclase-gabbros. 



AGE OF CAMBRIAN BEDS RESTING ON LAKE SUPERIOR SERIES 



The fauna of the Cambrian sandstone which comes in contact with 

 the Algonkian rocks in the vicinity of Saint Croix falls is the type of 

 that of the middle Cambrian, and it is probable that by overla]) the 

 higher beds of the Cambrian also come in contact with the rocks of the 

 Keweenaw series. 



UNCONFORMITY BETWEEN LAKE SUPERIOR SERIES AND CAMBRIAN 



A full description of the relations of the Cambrian to the Keweenaw 

 rocks is given by Irving in his monograph on the copper-bearing rocks 

 of lake Superior.* He shows clearly that there is a great unconformity 

 between the two series of strata, and that profound orographic movements 

 and long continued erosion occurred prior to the deposition of the C'am- 

 brian rocks (figure 7). 



UNCONFORMITIES WITHIN LAKE SUPERIOR SERIES 



The unconformities between the Keweenawan and the Upper Huro- 

 nian, and between the Upper Huronian and the Lower Huronian, and 

 between the Lower Huronian and the Archean have been fully described 

 and discussed by Van Hise, f who states that 



" In the lake Superior region, between the Archean and the Potsdam sandstone, 

 the great Algonkian system is subdivided into three series, which are separated bj' 

 very considerable unconformities. The lowest series is closely folded, semicrystal- 

 line, and consists of limestones, quartzites, mica-slates, mica-schists, schist-con- 

 glomerates, and ferruginous and jaspery beds, intersected by basic dikes, and in 

 (;ei'tain areas also by acid eruptives. It includes volcanic elastics, often agglomer- 

 atic, and a green chloritic, finely laminated schist. The thickness of this series 

 has not been worked out with accuracy, but at its maximum it is probabl}^ more 

 than 5,000 feet. As the term Huronian has been for many years applied not only 

 to the Upper Huronian series, but to this inferior series about lake Superior, it is 

 called Lower Huronian. 



" Above this series is a more gently folded one of conglomerates, quartzites, 

 shales, slates, mica-schists, ferruginous beds, interhedded and cut by greenstones, 

 the whole having a maximum thickness of at least 12,000 feet. In the Animikie 

 district a fossil track has been found, and in the Minnesota quartzites lingula-like 

 forms, as well as an obscure trilobitic-looking impression. Carbonaceous shales 

 are abundant. In its volume, degree of folding, and little-altered character the 



*.Mon. U. S. Geol. Survey, vol. v, 1SS3, p. 306, jil. xxiii. 

 fHull. U. S. Geol. Survey, no. Sil, 1802, pp. 4ill)-50ii. 



