416 THE CRETACEOUS AND TEKTIARY GEOLOGY 



where the cretaceous beds are not limited on their inner margins liy such ranges do 

 they rest upon or against the cr3\staliine rocks. 



Denudation. — The present distribution of the tertiary, the horizontality of its 

 beds, and the high angles of their exposed mai-gins, all go to show that the denuda- 

 tion of these rocks has gone on and is still going on at a rapid rate, whatever the 

 causes. Erosion and denudation are still in progress, and some of the causes, at 

 least, may be studied upon the ground. Tlie tertiary beds frequently form broad, 

 dry, and sterile plateaux or table lands known in Brazil as " chapadas " and " tabo- 

 leiros." In places these plateaux are almost barren of vegetation, and are deeply 

 scored with gullies, ravines, and gorges as much as two hundred feet in depth. Such 

 spots are quite common about the mai'gins of these plateaux. It is not an uncommon 

 thing, however, for hills having more or less vegetation over them to yield to erosion 

 and to develop such eases as that shown in the accompanying illustration of the 

 " Ruinas de Palmyra," near Cabo Santo Agostinho. This case is a typical one (y. 

 Plate III). One is often struck, on looking at these rapidly cutting exposures of 

 tertiary, with their resemblance in miniature to the topography of parts of the Grand 

 Canon region of the United States. 



The great denudation of the tertiary beds is due to the following five principal 

 causes : 



I. The impenetrability of the surface clays. 



II. The sterility of the soil resulting in barrenness of vegetation, and the absence 

 of the protection arising therefrom {v. p. 370). 



III. The unresisting nature of the rocks {v. p. 410). 



IV. The great precipitation concentrated within a few months of the year in- 

 creasing in geometric ratio the abrading and transporting powers of the Avater. 



V. The temperature of the water considerably elevated by falling upon the bare 

 hot surface clays. 



I shall si)eak of these causes in their order. (I) The impenetrability of the soil 

 prevents the pi-ecipitation from soaking into the ground, obliges it to seek its level 

 along the surface at once and is therefore conducive to Hoods and to corrasion. The 

 tertiaiy clay through this region being more or less impenetrable, sheds its water 

 almost perfectly, and thus hastens erosion by floods and freshets whose abrading and 

 transporting powers are enormous. 



It will be observed that in almost all the descriptions of Brazilian tertiary sec- 

 tions there are alternate sandy and clayey beds. When erosion acts readily upon 

 the sandy ones they are soon removed until a bed containing sufficient clay to turn 

 the water readily is reached. This bed then becomes the surface clay of the country 



