1913.] REGION ABOUT NATAL, BRAZIL. 461 



The gaining of the dunes over the river is most probably done 

 in times of drought, for then they can fill the river, cross it, and be 

 carried into the valley. 



Economic Geology. 

 Building Stone. 

 The crystalline series of rocks contains good building stones, 

 such as granites. Far into the interior there are marbles, but they 

 are rather inaccessible now. Granite is now quarried at Macahyba 

 by the method of cooling the rocks suddenly by throwing water on 

 them after they have been heated by fire. On account of the 

 scarcity of large timber and its rapid decomposition due to climatic 

 conditions and the work of such insects as ants, building stone is an 

 important factor. The ordinary houses are usually made of mud 

 with a reenforcement of slender limbs of shrubs and trees. 



Lime. 

 The limestone series afl^ords a means of making lime and cement. 

 It is quarried where it is found exposed at the surface, but could be 

 obtained at many other places if sought for. The quarries contain 

 bedded rocks of varying economic value. Some of the beds are 

 mostly siliceous, while others are almost entirely composed of cal- 

 cium carbonate. Three specimens were dissolved in weak hydro- 

 chloric acid and gave the following percentage of insoluble matter, 

 varying from 4 per cent to over 20 per cent. 



Locality. Percentage of Insoluble Matter. 



Jacoca lo.s 



Masaranduba 3.9 



Itapasaroca 20.1 



The limestone belt, occurring next to the granitic series, affords 

 a means for obtaining cement to use in the same place in connection 

 with the building stone. 



Clay. 



The clay in the upper sandstone series is sometimes quite pure. 

 It is the material of which the native huts are made. In some places 



