19!8] Davis: The Radiolarian Cherts of the Franciscan Group 341 



subordinate amounts of cherty iron carbonate. This is based on the 

 observation that some of the least altered parts of the formation, 

 closely associated with slates, show much greenalite, and also that in 

 studying a large number of thin sections it was possible to trace all 

 gradations in the alteration of greenalite to iron ore. Many of the 

 jaspers showed small residual patches of greenalite rock. The cherty 

 iron carbonates have also been shown to pass over into iron ore. The 

 ferruginous cherts are believed to result from the alteration of greena- 

 lite rock or cherty iron carbonate. Iron ores come from further altera- 

 tion of ferruginous cherts. 



Van Hise and Leith 09 also admit the possibility that the ferrugi- 

 nous cherts and interbanded iron ores may have originally been de- 

 posited as banded silica and ferric oxide or ferric hydrate. 



As a result of the evidence which is fully presented and discussed 

 in their monograph, Van Hise and Leith reached the conclusion that 

 the iron formations were true marine sediments, though of an unusual 

 and exceptional type. They also concluded that the iron oxide and 

 silica were chemical precipitates from solution. 



They point out the frequent close association in space and time 

 (shown by interbedding) of the iron formations and the ellipsoidal 

 basalts which were extruded in enormous volumes in the region of 

 Lake Superior during various periods of pre-Cambrian time. They 

 also point to the association of these peculiar extrusives with radio- 

 larian cherts in many parts of the world. 



They regard the ellipsoidal basalts as the "variant in the normal 

 conditions of sedimentation necessary to produce the iron bearing 

 formations," and believe that in these basalts they can recognize 

 the source of the iron and silica. 



They believe the material may have been carried in hot solutions, 

 coming from the eruptive rocks during their crystallization, or leached 

 by meteoric waters from the portions of the magma extruded sub- 

 aerially, or, in small part, obtained by reactions between the iron rich 

 basalt and the sea-water, when the basaltic magma was poured out 

 on the sea floor. 



They attempt to meet the objection that the Animikie Iron forma- 

 tions on the north shore of Lake Superior, which represent the maxi- 

 mum development of iron formation in the district, are not directly 

 associated with volcanics of this type. They argue that the remark- 

 ably uniform character of these Animikie iron-bearing rocks, so dis- 

 tinct from pre-Cambrian iron formations, is due to distance from the 



»9 Op. cit., pp. 126, 243. 



