GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 397 



showing that, while the mechanical sediments consisted mainly of 

 quartz sand, clay also was frequently deposited in certain localities. 



Thus it is clear that in structure and general occurrence the 

 iron formation is so like ordinary sediments that there is little 

 doubt as to its sedimentary origin. It remains, however, to account 

 for its peculiar and unique composition and also for its enormous 

 mass. It is difhcult to picture a source for such an enormous 

 amount of iron oxide as is contained in the iron formation, and 

 even if an adequate source of supply be found, it is not so easy to 

 see how the iron oxide came to be deposited locally in masses 

 hundreds of meters in thickness with less than ^ of i per cent of 

 siliceous sediments intermixed. 



It is apparent from the composition of the Caraga quartzite 

 that during the first stages of the sedimentary deposition the pro- 

 cesses of decomposition greatly exceeded in effectiveness the proces- 

 ses of mechanical disruption, for, instead of arkose sediments, the 

 Caraga quartzite is made up of the products of mature weathering. 

 This may be ascribed to a long period of decomposition without 

 transportation, resulting in the accumulation of a thick mantle of 

 residual material on the land surface, or it may be ascribed to a 

 warm, moist climate, or to extensive wave action with practi- 

 cally continuous sedimentation. The material of which the sedi- 

 ments are composed was derived from the decomposition of rocks 

 of the basement complex, such as granite, gneiss, amphibolite, and 

 crystalline schists, with some basic intrusives. The principal 

 minerals associated in the make-up of these rocks are quartz, 

 feldspar, biotite, hornblende, and some pyroxene, whose end prod- 

 ucts after weathering are quartz, kaolin, hydrated iron oxide, and 

 some aluminum hydroxide. These four constituents, therefore, 

 probably made up the bulk of the decomposition products. 



During the sedimentation which followed the post-Archean 

 erosion period the first material to be deposited in central Minas 

 Geraes was quartz sand, from which it is natural to suppose that 

 the finer products of the decay of the basement complex — the 

 kaolin and hydrated iron oxide — were carried farther out to sea, 

 while the coarser quartz sand was deposited nearer the shore. 

 This process must have been long continued, for the Caraja 



