GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 349 



pegmatite veins are also abundant in the gneiss but are rarely seen 

 in the granite. They usually consist of muscovite, orthoclase, and 

 quartz. 



The amphibolite is a banded rock occurring interlayered in 

 the gneiss and corresponding in layering to the gneiss itself. It is a 

 dark-green rock consisting either entirely of hornblende, or of 

 hornblende with a small amount of feldspar. It is widely dis- 

 tributed throughout the region of the basement complex, and in 

 many localities masses of considerable thickness occupy large areas. 

 On weathering the amphibolite gives rise to a yellow ocherous soil 

 due to the formation of abundant iron oxide. Near the surface 

 this changes in color to a deep red which is very characteristic of the 

 soils in many parts of the district. 



Diorite and gabbro occur sparingly as later intrusions in the 

 gneiss and schist in different parts of the district, diorite being 

 more common than gabbro. Most of the intrusions are in the 

 form of dikes, some of which may be traced for long distances. 

 Some of the gabbro intrusions, however, are very limited in 

 extent. 



The relation of the three principal varieties of rock in the base- 

 ment complex to each other is determined with difficulty on account 

 of the complexity of their structure and distribution, and because 

 of the thick covering of mantle rock which effectively conceals 

 the greater portion of these rocks. The granite is apparently 

 intrusive into the gneiss and schist and occurs in large irregular 

 areas. Single masses 60 or 70 kilometers in extent are known. 

 The granite intrusions are probably older than the gabbro and 

 diorite. The relation between the schist and gneiss is unknown. 

 In places the schist has been seen interlayered with the gneiss and 

 apparently bears a relation to it similar to that which the amphibo- 

 lite bears to the gneiss. If this is true the schist, gneiss, and 

 amphibolite would seem to be the older rocks into which granite, 

 diorite, and gabbro were intruded. Probably the gneiss and 

 amphibolite represent respectively ancient acid and basic volcanic 

 flows with which were interbedded quartzose and argillaceous sedi- 

 ments now represented by the schists. By metamorphism and 

 deformation these were transformed into their present equivalents — 



