92 J. C. BRAN NER— GEOLOGY OF NORTHEAST COAST OF BRAZIL 



found where the old crystalline schists, gneisses, et cetera, are decayed 

 in place. It is therefore believed that some of the coloring of the Ter- 

 tiary sediments is due to the weathering in place of the crystalline rocks, 

 and that some of the Tertiary beds have therefore always been highly 

 colored. In other instances the coloring has taken place subsequent to 

 the deposition and elevation of the Tertiary beds. This is shown by 

 the size and character of the particolored areas. 



There is a notable topographic contrast between the region of Tertiary 

 sediments and the region of old crystalline rocks. The soft Tertiary 

 beds rise to an approximately even elevation and are cut by steep-sided, 

 narrow, closely spaced valleys or gorges, while the region of old rocks is 

 more rounded in outline and the valleys are broader and the elevated 

 watersheds farther apart. 



Geology along the Natal a Nova Cruz Railway 

 bail road stations and topography adjacent thereto 



The lower part of the city of Natal stands on an alluvial flat, only a 

 meter or two above high-tide level. This flat ends abruptly and sharply 

 against the slope of the ridges, as if its materials had been deposited in 

 water standing at that level against the hills. The " upper city " is on 

 a hill of red Tertiary sediments that rises some twenty meters or more 

 above high-tide level. The streams that cut across the region above or 

 inland from the city are bordered by broad mangrove swamps, while 

 the lowlands between the city and the sea is either mangrove or tide 

 marsh or is covered with sand dunes. 



At kilometer 3 the view over Rio Grande and the estuaries about 

 Natal throw light upon the topographic history of the region. The 

 water of the estuaries is bordered with mangrove swamps, and these 

 swamps merge into flat lands which end abruptly against the rather 

 steep slopes of the hills beyond. At kilometer 3 the railway is near the 

 general level of the Tertiary plateau that forms the rather narrow belt 

 along the Brazilian coast from Natal nearly to the mouth of the Rio Sao 

 Francisco. 



At Pitimbu (kilometer 12) the stream runs through a flat valley 

 filled with a black, muck-like soil between steep hills. 



From kilometer 15 to 19 the country is a somewhat rolling, but nearly 

 flat, dry plateau, with a very scant vegetation. 



The sand dunes that have blown up from the coast are visible from 

 the railway at many places across the Tertiary plateau. Where the road 

 approaches the coast they form a very striking feature of the topography — 



