464 JENKINS— GEOLOGY OF THE [May 29, 



The occurrence of granitic rocks does not afford the means of ob- 

 taining underground water as in the region of the sedimentaries 

 along the coast. 



Effects of Climate, Etc. 



All the ocean currents and the winds constantly sweep up the 

 coast towards the northwest. The water is warm and the atmos- 

 phere balmy. The greatest rains and the hottest weather occur dur- 

 ing the months of our winter. During our summer the rains are 

 more scarce and the atmosphere cooler and drier. The region about 

 Natal does not have such heavy rains as some of the other parts of 

 Brazil such as those nearer the equator. In fact the country about 

 Natal is very healthful. 



The direction of the winds and the ocean currents affects the 

 coast line in that all the bars, spits, reefs, and promontories tend to 

 point up the coast to the northwest. 



The stone reef at Natal affords the maintenance of a quiet 

 harbor. 



Effect of a Sunken Coast. 



A sunken coast affords fine harbors. Cities occur at the points 

 where arms of the sea extend up the river channels. Natal is a 

 good example of such a city beside a harbor thus formed. The 

 tides which flow up and down these channels give to the people an 

 easy method of transportation. 



Effect of Silting. 



A constant annoyance to the people is the perpetual silting up of 

 the channels and the shifting of sands and the formation of sand 

 bars in their harbors. These things change rapidly, being dependent 

 largely on the amount of rainfall, the flow of the rivers, and the in- 

 tensity and direction of local currents. 



Mangroves help the silting process to a great degree, but still 

 these swamps when drained finally make up the fertile lands of the 

 country. 



The depth of the silt in the river channels often causes disturb- 

 ance to the people when they attempt to drive piles in the mud and 

 cannot find bed rock. 



