Waring: Geology of Northeastern Brazil. 221 



is near Assu, four hundred kilometers to the southeast, where R. H. 

 Soper found an area about one kilometer in diameter, dotted with 

 basaltic boulders. A petrographic slide of a specimen of this rock, 

 which was given to the writer by Mr. Soper, was examined by E. S. 

 Larsen, Jr., who found the rock to be an ordinary basalt, with abund- 

 ant large, slightly altered crystals of olivine. Plagioclase is plentiful 

 as labradorite and there is considerable augite in small rods. Mag- 

 netite is also plentiful and some apatite is present. The matrix is 

 pale yellow to colorless, part isotropic, part polarizing, probably glass. 



Acid lavas are also very rare in the region. On the coast of Per- 

 nambuco Dr. Branner found trachyte at a headland known as Pedras 

 Pretas, twenty-five kilometers south of Recife, and rhyolite on the 

 little island of Santo Aleixo, thirty kilometers farther south. ^^ Of the 

 former he says "I recall no other occurrence of trachyte in Brazil." 



Springs. The scarcity of springs in most of northeastern Brazil is 

 notable. Even mere seepages, such as afford supply to prospectors 

 and travelers in the desert portions of the southwestern United States, 

 are rare in the great areas of crystalline rocks in northeastern Brazil. 

 Throughout these areas, in localities away from the shallow under- 

 ground supplies of the wide, sandy stream-channels, dependence is 

 placed upon shallow reservoirs formed by earthen dams across the 

 courses of small streams, or in laboriously excavated 'tanques,' which 

 are also supplied by surface run-off. Within the crystalline areas 

 only three ordinary or "hillside" springs were seen by the writer, 

 near the border of Campina Grande, at Olho d'Agua (Spring) two 

 kilometers north of Viragao, and at Pau dos Ferros, a settlement of 

 six or eight houses four kilometers north of Periquitos. All three 

 springs are mere tricklets, which issue from the more or less crushed 

 rocks of the Planalto da Borborema. 



The great storage reservoirs formed by the Cretaceous sandstones 

 of the highland areas of Serra Grande, Chapada do Araripe, Chapada 

 do Apody, and minor areas, supply a number of springs, which issue 

 from the bluffs; but not to the extent that might be expected. The 

 city of Ipu, at the base of Serra Grande (Plate VIII, fig. 2) is supplied 

 by springs which issue near the top of the range, two hundred and fifty 

 meters above the city, and cascade down with a flow of from one 

 hundred and fifty to two hundred liters per minute. Similar springs 



^o Geology of the Northeast Coast of Brazil, pp. 59-60 and 62. 



