3S8 E. C. HARDER AND R. T. CHAM BERLIN 



Itahira iron formation. — The Batatal schist represents a slacken- 

 ing of sedimentation from the rapid deposition which characterized 

 the laying down of the sands composing the Carafa quartzite. 

 This slackening of clastic sedimentation continued until the close 

 of the Batatal epoch, when very little clastic material was being 

 washed into the sea in the region considered. The land pre- 

 sumably had become so low as to yield very little mechanical sedi- 

 ment, and with this lowering of the land surface there was probably 

 combined a gradual retreat of the shoreline. Simultaneously with 

 the great diminution in mechanical sediment deposited in the area 

 under consideration, there commenced a precipitation of ferric 

 hydroxide from solution, materials in solution being probably car- 

 ried beyond the borders of the region of clastic sedimentation. This 

 precipitation may have been due, either to purely chemical reactions 

 taking place in the sea, or perhaps to the operation of the well- 

 known iron bacteria which cause the deposition of ferric hydroxide 

 from waters containing ferrous carbonate in solution. These 

 iron bacteria are said to possess the peculiar property of utilizing 

 as food the carbon dioxide locked up in very dilute solutions of 

 ferrous carbonate.^ Ferric hydroxide is left behind and is deposited 

 as a sediment. The characteristic reaction may be written: 

 2FeC03+3H,0+0=2Fe(OH)3-f2C02. This process is operative 

 in very dilute solutions. Apparently only two or three parts of 

 iron per million are needed to make certain types of the iron 

 bacteria active. 



Not having much confidence in the hypothesis that the iron 

 oxide was precipitated directly from sea-water by ordinary chemical 

 means, we prefer to turn to the iron bacteria as perhaps forming 

 a better working hypothesis. If such a process as this be sup- 

 posed to have been steadily in operation at this time, when very 

 little clastic sediment was being deposited, it might have resulted 

 in the production of the extensive iron formation. For reasons 

 which will be discussed later, this hypothesis seems to explain the 



^ S. Winogradski, "tjber Eisenbakterien," Boian. Zeitung, Bd. 46 (1888), p. 261; 

 Rudolf Lieske, "Beitrage zur Kenntnis der Physiologie von Spirophyllum ferrugineum 

 Ellis, einem typischen Eisenbakterium," Jahrh. fiir wissenschaftlicher Botanik, 

 Bd. 79 (1911), pp. 91-127. 



