342 



University of California Publications in Geology [Vol. 11 



center of volcanic activity. They also point out that rocks referred 

 to the Animikie on the east side of Hudson Bay show all the members 

 of the ordinary iron formations of Lake Superior and are also inti- 

 mately associated and interbedded with extrusions of ellipsoidal 

 basalt. This indicates that this type of vuleanism was a feature of 

 Animikie time. 



The cherts and jaspers of Lake Superior may be contrasted with 

 the cherts of the Franciscan formation by reason of their much higher 

 iron content, their lack of radiolaria, and the presence of the peculiar 

 greenalite granules. The absence of radiolaria might be regarded 

 as due to the greater metamorphism of these older rocks. However, 

 in some places the slates appear to have escaped unusual metamor- 

 phism, and in view of the great number of careful workers, and the 

 large number of thin sections examined, it would seem that if the 

 rocks had ever contained radiolaria in important amounts, some trace 

 of them would have been discovered. It seems most reasonable to 

 regard them as siliceous rocks not associated with radiolaria in any 

 way. 



In so far as the ferruginous cherts of Lake Superior are due to 

 the alteration of greenalite rock and of cherty iron carbonate they 

 appear of a distinct type from the Franciscan cherts. The red cherts 

 of the Franciscan group show no evidence of alteration and appear 

 to be the result of original sedimentation. 



Jaspers op South Africa 



Jaspers, associated with iron ore, occur at several horizons, and 

 at several localities in South Africa. These all resemble very greatly 

 the occurrences in the Lake Superior District and need not therefore 

 be described in detail. 100 



These iron formations are intimately associated with basic igneous 

 rocks, both extrusive and intrusive. In many cases these show an 

 ellipsoidal structure. No radiolaria have been reported from them, 

 though they have not been studied in the same intensive manner as 

 the Lake Superior rocks. Unlike the cherts and jaspers of Lake 

 Superior, these rocks in South Africa are frequently associated with 

 the blue soda amphibole, crocidolite, which in certain places occurs 

 in abundance. 



100 Hatch and Corstorphine, The Geology of South Africa, London, 1909. 

 Kogers and Dutoit, The C4eology of Cape Colony, London, 1909. 

 Stow, G. W., Geological Notes on Griqualand West, Quar. Jour. Geol. Soc, 

 vol. 30, p. 581, 1874. 



