374 THE CRETACEOUS AND TERTIARY GEOLOGY 



by the Rio Sergipe, and their relations to each other and to the underlying archaean 

 rocks are distinctly shown. 



Aside from its purely geologic interest and importance, there is, perhaps, no part 

 of the coast north of Bahia so fertile, the soil being of the black quality known here 

 as massape and deiived from the decomposition of certain limestones and of organic 

 matter. Whei-evcr the tertiary beds have been eroded away exposing the underlying 

 cretaceous rocks, the decaying of these calcareous beds has produced a soil, which, in 

 spite of indifferent cultivation, yields abundant crops of sugar-cane, cotton, mandioca, 

 etc. Although the cane-fields upon some of the cretaceous soils have not been re- 

 planted for thirty years, they still produce abundantly. 



TOPOGRAPHIC FEATUKES AND VEGETATION. 



The topography of the Sergipe- Alaguas region may be divided into the following 

 five types : 



I. The low, flat lands of quaternary and recent origin, immediately bordering 

 the coast and streams. 



II. The tertiary plateaux. 



III. The cretaceous hills. - 



lY. The serras, or paleozoic region. 

 Y. The ti-ans-serra, oi- archseau region. 



I. The low, flat lanch. — In some places these flat lands are made up of sand bar- 

 rens, or they are covered with sand dunes ; in others they are low grounds flooded at 

 high tide, and usually form what ai-e known as mangues, or mangrove swamps, bor- 

 dering the estuaries. This type, or rather the mangi'ove part of it, is not confined to 

 the immediate neighborhood of the ocean, but extends inland as far as the tides are 

 felt, sometimes for many miles, and in general outline has a dendritic form. On the 

 Cotinguiba it reaches the town of Larangeiras, on the Sergipe tliat of Riachuello, 

 and on the Maroim branch it reaches the city of that name. 



The vegetation of this belt is a characteristic one. The mangues are in no par- 

 ticular different from those to be found along the whole Brazilian coast, except, pei- 

 haps, in the blackness of the mud where it is derived from the decomposition of the 

 calcareous cretaceous rocks and organic matter. In places these mangues broaden 

 out until they are from two to five miles wide, and in other places their outer margins 

 approach the streams where the latter are encroached upon b}^ the hills. Thus they 

 are at once the characteristic feature of a well-defined topographic type, an important 

 geologic agent, and an interesting form of vegetation. When the mangrove plants 



