branner: the stone reefs of brazil. 35 



He notes that a soft conglomerate is found in the bottom of the harbor 

 behind the reef. 



Borings were made on the reef showing the rock to be not much more 

 than a metre thick. It is said that there was formed a channel between 

 the reef and the shore. 



Tlie stone reef at Rio Grande do Norte. — The immediate shores about 

 the mouth of the Rio Grande do Norte are covered far and wide with 

 shifting sands. Southeast of the city these sands are being blown 

 over the hills and into the river to such an extent as to threaten to 

 destroy the navigability of the stream between the city of Natal and the 

 bar. Behind these dunes mangrove swamps spread out across the mud 

 flats that follow the river as far up as the tides are felt. 



The topography of the region is beautifully seen from kilometre 3 of the 

 Natal a Nova Cruz Railway. From this point one looks down upon a 

 broad flat valley where water merges into mangrove swamp and swamp 

 into flat dry lands, all ending abruptly against the sedimentary hills to 

 the north. This flat country continues westward up the estuary of the 

 Jundiahy to the town of Macahyba, everywhere the same as far as the 

 general features are concerned. 



In the city of Natal itself the same topographic relations are visible 

 in the open square in front of the railway station. 



..ssimiisA. 



Fig. 12. Section from Natal to the Stone Eeef. 



It is worthy of note that the reef is on a level with the mangrove 

 swamps, and but slightly below the level of the land upon which the 

 lower city stands. Tlie Tertiary hills on which the upper part of the 

 city stands plunge as abruptly beneath the flat land at their base as if 

 that land were a water surfixce. 



The Natal or Rio Grande do Norte stone reef connects with the shore 

 at its southern end, while its northern end stands squarely across the 

 mouth of the Potengui, or Rio Grande, so that the water of that stream 

 flows round the north end of the reef to escape to the sea. The length 

 of the reef from its northern end at the bar to where it joins the land 

 is fifteen hundred metres ; its length from where it joins the land to its 

 southern extremity is three kilometres, making a total length of four 



