I9I3] 



REGION ABOUT NATAL, BRAZIL. 



439 



being deposited along the coast, blown inland by the wind. The 

 sand tends to fill up the river systems, for they are the lowest places. 

 In time of drought the sand sometimes is able to gain complete 

 control over some of the streams, stopping them up entirely. In 

 time of great floods the water clears its channel again. A good 

 example of damming by sand is the valley of Extremoz, where 



Columnar Section 



Description 



Recent 



Quaternary ^ 



SanoL — dunes. 



Valley alluviunx, marine and 

 ■*^(bracKisVi wokter shells. 



Pcarticoloreci sonnds^or^cs a.na( 



Pole07.0i(.(?) 



and 

 Archean 



Lirnesi"on©& arxd ^andtj limestones, 

 tna-Tine and brackish vvofer- ^Viella^ 

 ond aorne leaver. 







x-^-t^-J 





Crystalline rocKs: 

 granit-es, 3nciS5C6, &ohi*t^, 

 cjjuartzites, slates, marbles, 

 e+c. 



Fig. 5. 



sand-dunes are heaped in the valley in longitudinal rows, while 

 at its mouth is a great sand bank, twenty-five to thirty meters in 

 height, lying across it at right angles. About fourteen kilometers 

 inland Lake Extremoz lies in the forks of the old river channel, 

 dammed in by this wind-blown intrusion. 



Description of the Contacts, 



The limestones lie unconformably on top of the old crystalline 

 series. The contact was plainly seen at Alvoredo, about five kilo- 

 meters north of Macahyba, in the bed of the Potengy River. 



The sandstone and clay series lies unconformably on top of 

 the limestones. At Jacoca, five kilometers southwest of Ceara- 

 Mirim, the contact was observed in the limestone quarries. This 

 showed the fossiliferous limestones in clearly defined beds, six 

 inches or a foot thick, lying horizontally, with the sandstones rest- 



