GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 373 



material has been washed down on to lower benches and terraces 

 along the courses of some of the streams which dissect the plateau, 

 and today diamond mining operations are also carried on at these 

 lower levels. 



LATER EROSION ANB DEPOSITION PERIODS 



General uplift. — As to the age of the peneplain nothing definite is 

 known, but it would seem most likely to belong to the late Mesozoic 

 or early Tertiary, At some time subsequent to the formation of the 

 plain, the region was uplifted as a whole with but little change in the 

 attitude of the strata. Rejuvenated erosion then commenced 

 to carve the present mountains out of the uplifted plateau. The 

 resistant quartzites remained as ridges, while the softer schists 

 and readily disintegrated granites and gneisses were more rapidly 

 worn down and removed. In this tropical country the granites 

 and gneisses appear to be the least durable of the formations, owing 

 to the rapidity with which they crumble to pieces following the 

 weathering of their ferromagnesian constituents. The micas, 

 amphiboles, and pyroxenes are readily attacked by humic acids 

 and by the carbon dioxide of the atmosphere. Weathering of 

 these constituents allows the crystalline mass to crumble into arkose 

 which is removed by the streams. The result is that in Minas 

 Geraes the areas of granite, because of the feeble resistance of this 

 kind of rock to the accelerated chemical action characteristic of 

 weathering in the tropics, have become the lowlands. Some 

 prominent granite ridges occur, but on examination it is generally 

 found that they are composed of light-colored granite in which the 

 ferromagnesian constituents play a subordinate part. The quartz- 

 ite formations in general owe their resistance to erosion mainly to 

 the fact that they contain a very small percentage of readily decom- 

 posable minerals. 



Tertiary canga plains. — The erosive processes which have carved 

 the mountains of central Minas Geraes from the uplifted peneplain 

 did not go on uninterruptedly. There were times of checked ero- 

 sion, locally at least, followed by rejuvenation. These lesser cycles 

 are shown by various local plains and remnants of plains in 

 different parts of the region. One of the most notable of these is 



