160 J. C. Branner — Geology of Fernando de Noronha. 



ments of a great variety of other rocks. The sea has carved 

 out at the base of its cliffs a beautiful and regular bench 

 from twenty to forty feet in width, which runs across the 

 whole western end of the Sapato midway between high and 

 low tides. The beds here dip westward at an angle of about 

 25 degrees. At the northwestern corner of this exposure a 

 stratum of compact rock (basalt?) about fifteen feet in thick- 

 ness overlies the tuff and dips 40° to the eastward. Above 

 this is a bed of very hard but much shattered rock which 

 continues to and beyond the Portao of which it forms the roof. 

 Basaltic tuffs, very similar to or perhaps identical with those 

 in which the Portao opening is excavated continue along the 

 north shore of the island for at least half a mile east of the 

 Portao. The cliffs of this material are usually vertical and 

 capped by a bed of some more resisting rock. From the prom- 

 ontory called Portaozinho, looking northeast along the trend 

 of the island's north coast, one sees rising abruptly from the 

 water a lofty vertical cliff of what appears to be rudely colum- 

 nar basalt. This exposure was not examined near at hand. 

 Both east and west of this exposure are others of reddish 

 brown rocks resembling in general appearance, and at a dis- 

 tance, the soft reddish palagonite of Barro Yermelho (No. 62). 

 The cliffs of this material have their bases deeply underscored 

 by wave action. 



Calcareous sandstone. — Besides the rocks of igneous origin, 

 a calcareous sandstone occurs along some shores. It covers 

 about one-third of Ilha Rapta, a part of Sao Jose, and small 

 areas of the main island near the Lancha on the northeast, the 

 high shore west of Atalaia Grande, the shore along the south- 

 west side of the Sueste Bay, and forms Ilha Raza, Ilha do 

 Meio and the Chapeo at the mouth of Bahia do Sueste. The 

 material of the sand-rocks was originally deposited in the form 

 of sand dunes and the bedding shows that it must have been 

 blown by the winds chiefly from a southern or southeastern 

 direction. The deposits are all on eastern or southeastern 

 shores and have no connection with the existing beach. On Ilha 

 Raza, it makes a perpendicular bluff forty feet or more in 

 height ; on Ilha Rapta it rises about forty feet above the 

 water, while south of Atalaia Grande it stands at an elevation 

 of at least a hundred feet above the sea. A microscopic ex- 

 amination shows that it has been consolidated by the deposit of 

 lime carbonate dissolved from the uppermost layers by the 

 waters of the rains aided possibly by the spray of the surf. 

 The grains are fragments of shells, corals, sea urchins, fora- 

 minifers and other calcareous growths of the shores. 



Where these sandstones rise from the ocean, as they do on 

 Ilha do Meio, Ilha Raza, Ilha Rapta and Chapeo, the wind 



