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E. C. HARDER AND R. T. CHAMBERLIN 



Such a process as this would seem to ojffer a possible clue to the 

 origin of the Itabira iron formation. As these bacteria thrive best 

 in clear waters, the low proportion of clastic sediment in the 

 formation is natural enough. If the deposition in general took place 

 in lagoons and embayments and the thicker and more siliceous 

 portions of the formation developed near the river mouths, in 

 regions of currents, or other favorable localities, and the thinner, 

 more uniform portions of the formation represent deposits formed 



Fig. 17. — A view of the Peak of Conceigao near Itabira de Matto Dentro, from 

 the summit of Caue. The iron formation forms a continuous belt between Caue and 

 Conceigao. 



farther off shore or in quiet waters near shore, the great and sudden 

 variations in the thickness of the iron formation at various points 

 may perhaps be accounted for. Much of the ferric hydroxide may 

 have been formed as a fiocculent precipitate in the sluggish river 

 waters carrying little or no clastic sediment, and later deposited in 

 a thick series of delta beds at the debouchures of the streams. 

 Such may be the great accumulations of iron formation at Alegria, 

 Gandarella, and elsewhere. Whatever the nature of this sedimenta- 



