362 E. C. HARDER AND R. T. CHAMBERLIN 



layers. But now its use is being restricted, and the name is applied 

 only in a petrographic sense to the peculiar iron-oxide bearing 

 quartzite which constitutes the sandy portion of the iron formation.^ 

 There is, however, a complete gradation between the itabirite and 

 the ore. 



Piracicaha formation. — After a period of very little wash from 

 the land, during which the iron formation was laid down through 

 rapid precipitation chiefly, an epoch of more abundant clastic 

 sedimentation again set in. This change was probably brought 

 about by a gentle uplifting of the land relative to the sea-level. 

 At first only the finer muds were laid down in the region under 

 consideration; later, with changing conditions, there were oscilla- 

 tions both in the direction of diminishing clastic sedimentation and 

 a return to the iron-oxide and calcium-carbonate forming conditions, 

 and also in the direction of increasingly coarse clastic sedimenta- 

 tion with the deposition of what has since become quartzite. Under 

 these fluctuating conditions which followed the deposition of the 

 Itabira iron formation there was laid down a great mass of sediment, 

 in which clays and muds predominated, but within which there 

 were included quite extensive lenses and more or less irregular 

 beds of quartz sand, iron oxide, and also calcium-carbonate 

 deposits. Through subsequent metamorphism these sediments 

 were altered to argillaceous and quartzite schist, quartzite, iron 

 formation, and limestone. The name Piracicaba formation has 

 been given to this series from the Piracicaba River, whose upper 

 course near Santa Rita Durao and in Alegria follows the strike of 

 these beds. 



The lenses and beds of limestone and iron formation occur 

 principally in the lower part of the Piracicaba formation, being 

 interlayered with schists, while the quartzite beds predominate in 

 the upper part. Limestone lenses, though usually small, are 

 numerous in many parts of the district, especially in the central 

 and southwestern parts. The limestone is generally impure, con- 

 taining, besides calcium carbonate, carbonates of magnesium, iron, 



' The term itabirite is coming into more general usage as is shown by the fact that 

 it has been applied to iron-oxide bearing quartzites occurring in certain parts of 

 Europe. See F. Beyschlag, J. H. L. Vogt, P. Krusch, Ore Deposits, I, 113, 194 

 (translation by S. J. Truscott). 



