202 Grout and Broderick — The Huronian. 



It is noteworthy that the structures appear in the 

 Mesabi range mostly in ferruginous cherts. Most of the 

 algal and similar organic structures in pre-Cambrian 

 rocks, so far described, are in carbonate rock. It might 

 be suggested that the original carbonate has in this case 

 been replaced by chert, but the lack of any remnant of 

 compact carbonate is opposed to the suggestion. The 

 carbonate found in associated beds is disseminated and 

 not apparently the remnant of a bed that was highly cal- 



Fig. 3. 



Fig. 3. Sketch in photograph of horizontal section through upper algal 

 beds (natural size) ; Biwabik formation. 



careous. Furthermore, there is no reason to expect 

 early forms to be so uniformly calcareous as recent 

 forms. And even recent organic precipitates include 

 some that are siliceous and some that are ferruginous. 

 Clarke has argued that the early forms were probably 

 dominantly siliceous. 5 



Forms and Structures. 



The structures and forms in the two beds are so differ- 

 ent that they require separate description. The average 



5 Clarke, F. W. : Geochemical evidence as to the early forms of life. 

 Jour. Wash. Acad. Sci., 6, 603, 1916. 



