GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 351 



thickened as erosion and transportation decreased and as it thick- 

 ened it became more and more thoroughly decomposed. 



While there were still considerable differences of elevation the 

 sea began to encroach upon the land, gradually occupying what is 

 now eastern, southern, and central Minas Geraes. Apparently an 

 arm of the sea extended westward covering the southern end of 

 Goyaz, the southern and western part of Matto Grosso, and prob- 

 ably part of Bolivia. Whether the sea extended far southward 

 into Sao Paulo is not known, but it is doubtful, since sediments of 

 this age are unknown in central and southern Sao Paulo. To the 

 north the sea covered northeastern Minas Geraes and a large part 

 of Bahia, but whether it extended still farther north is not known. 



It was in this sea that the probable Algonkian sediments of 

 central Brazil were deposited. Owing to the great amount of 

 decomposition which the rocks of the basement complex had under- 

 gone previous to the encroachment of the sea, little remained in 

 the soil except the end products, especially kaolin, iron oxide, and 

 silica. As the end products were derived largely from acidic 

 granites and gneisses, silica dominated. In the shallow advancing 

 sea the waves may be supposed to have been capable of sifting 

 these residual materials, holding in suspension and carrying away 

 the fine mud and iron oxide while of necessity soon depositing the 

 coarser grains of quartz. The first material laid down upon the 

 eroded surface of the Archean complex in the area under discussion 

 was therefore a great mass of quartz sand. By cementation, indura- 

 tion, and metamorphism this has become the Caraga quartzite 

 formation. 



PROBABLE ALGONKIAN 



Caraqa quartzite. — The Caraga quartzite is the basal formation 

 of the thick sedimentary series of supposed Algonkian age. Over 

 most of the region the formation consists of more or less schistose 

 quartzite, though it embraces a complete range from pure vitreous 

 quartzite to quartzite schist made up almost entirely of white 

 mica. The variations in composition and texture occur along the 

 beds, as well as across them, so that at different places along the 

 quartzite belts the major part of the formation may be either pre- 

 dominantly schistose or predominantly quartzitic. Locally the 



