GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 377 



As no attempt was made to correlate erosion cycles in the 

 different parts of the district, little can be said of these upward 

 movements of the land surface. One such uplift of the plateau- 

 forming type which affected the coastal region of Brazil in this 

 latitude was that which has given rise to the Serra do Mar, or 

 Coast Range. The effects of this movement were undoubtedly felt 

 by the drainage systems farther inland in Minas Geraes. The Serra 

 do Mar presents a very abrupt face toward the sea. At many points 

 it is really a plateau scarp which has the appearance of a mountain 

 range when viewed from the coast. Judging from the abruptness 

 of this plateau scarp, the upward movement which produced it 

 must have occurred late in the Tertiary, or possibly even early in the 

 Pleistocene. 



Present topography. — The present topography of central Minas 

 Geraes has resulted, to a very large extent, from differential erosion 

 of rock formations which offer varying resistance to the processes of 

 degradation. The mountains and ridges, valleys and plains, have 

 been carved out of a region which has been folded and faulted and 

 afterward reduced to base-level. The location of the present 

 topographic features has been determined by the underlying rock 

 structure. Wherever, because of faulting or folding, the heavy 

 quartzite formations appeared at the surface of the plain out of 

 which the present topography was sculptured, they resisted erosion 

 far more than the other rocks, and now remain as mountain ridges. 

 The iron formation, at most points somewhat less resistant than 

 the quartzite, but throughout most of its extent more lasting than 

 adjacent schists, stands up as a chain of foothills in approximately 

 parallel arrangement to the higher quartzite ridges. The schists, 

 by yielding more readily to erosion, have developed comparative 

 lowlands in the midst of which isolated ridges appear wherever a 

 lens of quartzite, or hard iron formation, occurs. 



But the conspicuously weak formation is the basement complex. 

 The granites and gneisses are especially susceptible to the chemical 

 disintegration which is induced by the tropical conditions. Their 

 ferromagnesian constituents are so readily attacked by the carbon 

 dioxide of the atmosphere and the humic acids from the rank vege- 

 tation, aided by the hot, humid climate, that the rock rapidly 



