GENERAL AND ECONOMIC GEOLOGY 



293 



These sandstone reefs have been in process of formation since Pliocene 

 times and are still being formed. The accompanying cross-sections of 

 the reef at Eia Formoso show their general structure along the entire 

 coast. 



Economic geology. — Beautiful crystalline marbles are found at many 

 places in the Archean areas of the state, notably at Sao Caetano, Floresta, 

 Pajeu de Flores, and Aguas Bellas. Some of these marbles are pure 

 limestones, and are much used in the manufacture of lime for local con- 

 sumption, while others are dolomites. The rhyolites of the island of 

 Santo Aleixo and the phonolites of Fernando de Noronha have been ex- 

 tensively quarried for street paving blocks. The sandstones of the reefs 

 were formerly much used for building stones and for paving sidewalks in 

 the city of Eecife. The use of these sandstones, however, has been wisely 



Figure 18. — Sections across a typical Sandstone Reef at Rio Formoso, 

 State of Pernambuco 



The ocean is on the right. The horizontal lines represent high and low tide. 



prohibited on account of the protection afforded the ports and the coast 

 by the stone reefs. 



Limestones of Cretaceous age are abundant in the Serra do Araripe, 

 in the northwestern corner of the state. At Maria Farinha on the coast 

 north of Olinda, limestones are found at the base of the Mesozoic sedi- 

 ments. 



One may often hear of the existence of rock-salt in the mountains about 

 Buique. The salt that has been made at Buique, however, is obtained by 

 leaching the dry earth or efflorescence gathered from certain sandstones 

 and evaporating the waters. 



Saltpeter is also found in the Buique region, but the deposits are 

 formed only on certain porous sandstones where the efflorescence is 

 scraped from rock surfaces on which it is formed by evaporation."^^ 



''^ L. Lombard : Revista Industrial de Minas Geraes, 

 de Junho de 1897. 



vol. V, pp. 6-7. Ouro Preto, 20 



