GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 361 



of such a thick deposit are avoided. In the eastern part of the 

 district, as along the Serra da Boa Morte, there seems to be much 

 more uniformity in the thickness of the formation. 



In chemical composition the iron formation also varies greatly, 

 and the changes are as sudden as the variations in thickness. In 

 some places, the entire thickness of the formation may be almost 

 pure iron oxide containing less than i per cent of impurity. At 

 other points there may have been so much arenaceous material 

 incorporated within the iron sediments that the proportion of 

 metallic iron is reduced to 40 per cent, or even less. But whether 

 the percentage of iron be high or low, the impurities consist largely 

 of rounded grains of quartz sand together with a little phosphorus, 

 the mineralogical relations of which are not known. In places, 

 however, the iron oxide is mixed with clay rather than sand, and 

 beds of ferruginous schist are interlayered with sandy iron forma- 

 tion. Thus, while ferric hydroxide was being precipitated, a 

 certain amount of sandy sediment, and locally, also, clay were 

 washed in to mingle with the precipitate. The presence of impure 

 limestone beds in the lower part of the iron formation, as southeast 

 of Bello Horizonte, shows that carbonate precipitation locally 

 accompanied the iron-oxide precipitation. In some portions of 

 the formation, the sand is more or less evenly disseminated, while 

 in others it occurs more often as distinct sandy partings, or laminae, 

 which separate thin beds of relatively pure iron oxide. The 

 temporary inwash of clastic sediment in the form of sandy partings 

 probably marks unusual storms, or stormy periods, during which 

 more of the terrigenous material found its way beyond the borders 

 of general clastic sedimentation. The rock breaks more readily 

 along the planes of these sandy partings than elsewhere, and hence 

 possesses a somewhat sandy fracture face, so that it appears to con- 

 tain more sand than is actually the case. 



The purer portions of the iron formation, where the proportion 

 of metallic iron exceeds 50 per cent, are regarded as iron ore, while 

 the more sandy portions of the formation are designated itabirite. 

 Formerly the term itabirite was applied as a formation name to the 

 entire iron formation member of the sedimentary series, and 

 included the iron ore as well as the sandy portions and schistose 



