GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 363 



and manganese. Different lenses differ in composition, some being 

 reddish or purplish in color and containing manganese carbonate 

 as the principal impurity, while others are grayish or brownish and 

 contain iron carbonate as impurity. In places thin layers of 

 itabirite occur in the limestone lenses, or considerable thicknesses 

 of interlayered limestone and itabirite may occur. 



The iron formation lenses in the Piracicaba formation are 

 generally more extensive than the limestone lenses but are more 

 local in their distribution. In lithology they are similar to the 

 Itabira iron formation, consisting mainly of itabirite but containing 

 also lenses of pure hematite and beds of ferruginous schist. In 

 most cases the iron formation lenses stand up as ridges above the 

 inclosing schist. 



The schists making up the lower part of the Piracicaba forma- 

 tion are mainly argillaceous, gray, red, or purple in color. In the 

 upper part sericitic, talcose, and quartzose schists are more common. 



Next to the Caraga quartzite the Piracicaba formation is 

 probably the most widespread of the metamorphosed sedimentary 

 rocks. It is almost coextensive with the former, except in the 

 northern part of the district where it is absent in many places 

 on account of faulting, but on the other hand it is present in places 

 where the Caraga quartzite is absent, as in the region southeast 

 of Bello Horizonte. 



Like the other sedimentary rocks, the Piracicaba formation is 

 of variable thickness. Furthermore it is not always easy to draw 

 the division line between this formation and the overlying Itacolumi 

 quartzite, since beds of quartzite become increasingly prominent, 

 in the upper portion of the Piracicaba formation. And as the 

 Itacolumi quartzite is represented only at a comparatively few 

 points in the district, the full thickness of the Piracicaba formation 

 is to be observed only in a limited number of places. But judging 

 from the exposures where the entire formation is present, it would 

 seem to be seldom less than 300 meters in thickness and generally 

 many times this. It is a great mass of sediment, of sufficient bulk 

 to lay a heavy tax upon nearly any system of dynamics and method 

 of sedimentation which may be supposed to have resulted in this 

 thick pile of beds. 



