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



the igneous rocks to gneiss and amphibolites, the sediments to 

 crystalHne schists. The later intrusions may have taken place 

 during, or after, the deformation. 



Many areas of schist, the definite structural relations of which 

 it has not been possible to determine, have been presumed to be 

 part of the basement complex. During further investigations 

 certain of these schists may be found to belong to one, or other, of 

 the overlying sedimentary formations. 



Lithologically the schists of the basement complex are extremely 

 various. There are quartz-muscovite schists, sericite schists, talc 

 schists, chlorite schists, amphibole schists, and argillaceous mica 

 schists of varying texture, many of them ferruginous, others sili- 

 ceous. The gneiss, on the other hand, is fairly regular in texture 

 and composition throughout most of the district. In most locali- 

 ties it is medium grained with distinct banding, though elsewhere 

 the banding is so faintly developed that it is difficult, by lithology 

 alone, to distinguish the gneiss from later granitic intrusives. 

 Its predominant minerals are feldspar, quartz, biotite, and horn- 

 blende. 



From the foregoing descriptions it is clear that to distinguish 

 between the various rocks of the basement complex, to separate 

 them areally and determine their structure and relations to each 

 other, will require much petrographic study and detailed mapping. 



POST-ARCHEAN EROSION PERIOD 



During the deformation and metamorphism of the gneiss, schist, 

 and amphibolite, and their intrusion by the granite and other 

 igneous rocks, the surface of the central Minas Geraes region was 

 probably of a very rugged and mountainous character, and was 

 probably elevated far above the surface of the sea. Then a period 

 of erosion and disintegration began, for a long time the former being 

 predominant, but later as the mountains were being worn down 

 and acquired gentler slopes disintegration became more and more 

 important. For a long time the products of disintegration and 

 decomposition were largely carried away to the sea, but gradually, 

 as the slopes became more gentle, material began to accumulate 

 and a layer or blanket of soil formed over the solid rock. This 



