454 JENKINS— GEOLOGY OF THE [May 29, 



to soft white ones, easily washed out by the sea. The harder por- 

 tions of the concentrated limonitic deposits often form irregular 

 shapes, leaving small holes and caves in which bats sometimes live. 

 These more resisting portions may sometimes be left standing out as 

 mere columns, breaking off before the beating of the waves. When 

 these irregular rough pieces collect together they cement one to 

 another and form a solid rocky point. So well is this cementing 

 process carried on, that it is often hard to find a single loose pebble 

 around a promontory. The soft leached parts of the cliffs are 

 washed away and thrown up on the beach, where the wind carries 

 them back over the countrv in the form of sand-dunes. 



Fig. 10. A typical section of the sandstone series as exposed in a stream cut 

 eight kilometers south of the fort at Natal, Rio Grande do Norte. 



All along the coast from Natal to Busios the bed rock which is 

 exposed is that of the sandstone series. The points named on the 

 map and many other smaller ones are all made up of hard red- 

 brown iron-cemented sandstones, which occur in irregular blocks 

 broken down from an adjoining low cliff. In some places, as about 

 eight kilometers south of the Fort at Natal and a hundred meters 

 from the beach, the sandstone series has been cut through by a 

 stream into deep, extremely narrow ravines. The beds are almost 

 horizontal and the general surface is level, but these peculiar cuts 

 break the surface, making it almost impassable although they are 

 not much more than ten meters deep. It is a sort of Grand Canyon 

 type of erosion. In such places bedding may be distinctly observed 

 and the nature of the strata easily studied. The beds vary in tex- 

 ture from a fine soft sand or a clayey sand to a coarse conglomerate. 

 These are interbeded, all carrying more or less clay, and varying in 



