376 THE CRETACEOUS AND TERTIARY GEOLOGY 



tion quickly take root and flourish upon it. The cacti are naturally in this list, and 

 I have found one species of fern here, while the caju tree, one of the blessings of 

 the country, seems to reach its highest development in this parched and arid soil. 



II. Tlte tevtianj j^lciteaux. — This and the succeeding types can best be under- 

 stood by anticipating somewhat the succession of geologic events in this region. The 

 cretaceous formation was here overlain by not less than two hundred feet of hori- 

 zontal, particolored beds of soft sandstones and clay. Since the emergence of the 

 region from the ocean, the overlying tertiary beds have been extensively eroded. 

 Where the tertiary beds have not been entirely washed away, flat-topped, angular- 

 shouldered hills still cover the cretaceous beds, and form the topographic type 

 under consideration. These plateaux are not confined to any particular belt, but 

 may be found anywhere between the ocean and the serras. The tertiary soils are, 

 as a rule, very sterile, their sterility being due partly and directly to the character 

 of the soil and partly to its tendency to wash, and its inability to receive and 

 retain sufficient moisture to support vegetation. The natural growth of vege- 

 tation upon this soil is strikingly poor, and this region of plateaux is 

 generally covered with a sparse and stunted growth of forest and sedge, while 

 the land is extremely barren, and unfit for cultivation. It is also seriously affected 

 by droughts. Types of this topography occur between Maroim and Sao Christovao. 



III. The cretaceous hills. — From what has been said it may be anticipated that 

 the region of the third type owes its existence to the erosion of the tertiary beds, and 

 the consequent uncovering of the underlying cretaceous. While this is true in the 

 main, there are certain elevations in this basin which I am disposed to regard as never 

 having been covered by the tertiary deposits, certaiiffy not to any considerable thick- 

 ness. In making a voyage between Aracaju and the mouth of the Rio Sao Francisco, 

 one may see a range of cretaceous hills lying south of the river and well inland, 

 though disconnected with the higher lands of the serras. This range of hills stands 

 at a higher elevation than any of the tertiary hills on either side of it, and for reasons 

 given further on, it is not believed that the erosion from the summits of some of the 

 tertiary liills has been very extensive. 



The region on the whole is a broken one, the hills being more or less rounded as 

 a rule, though a lew ol tliem have abrupt faces toward the north and west. Although 

 the topography of this region is a characteristic one, its value for the purposes of 

 geologic reconnoissance is somewhat impaired by the presence of the overlying ter- 

 tiary beds in a great many localities. This type characterizes the region between 

 Aracajii and Itabaiuna and between Larangeiras and the seri'as. The vegetation is 

 large and abundant, and within this belt one may often find excellent examples of the 



