62 J. C. BKANNEK — GEOLOGY OF NORTHEAST COAST OF BRAZIL 



secondary amphibole in a fine feldspathic groundmass. The rock has undergone 

 strong shearing. The feldspars are in part twinned on the albite law and one or. 

 two on the Carlsbad law, and they show an index of refraction greater than that 

 of the balsam. One quartz, apparently original, was noted. .The feldspars are 

 undoubtedly plagioclase, probably andesine. One grain of quartz that appeared 

 to be original was noted. There is magnetite present, and abundant minute sec- 

 ondary grains of undetermined nature." 



One of the rocks collected on the south side of Cabo Santo Agostinho, 

 where the relations of the granite, granite porphyry, and diorite to each 

 other is not altogether clear, is described by Mr Turner as a meta-rhyolite, 

 '' evidently original glass, now devitrified," an occurrence of interest in 

 connection with the rhyolites at Santo Aleixo. 



ROCKS OF SANTO ALEIXO 



The next rocks of especial interest are those of the little island of Santo 

 Aleixo, 30 kilometers south of Cabo Santo Agostinho. 



On the beach opposite and west of the island, and just southof Seramby 

 point, the sands are black instead of the usual straw color. As this is 

 the only place at which such sands were seen on more than 300 kilometers 

 of beach examined, it seems that these must have been derived from the 

 eruptive rocks of the island of Santo Aleixo opposite. Specimens of this 

 sand were collected, and Mr Turner, who kindly examined them for me, 

 says of them : 



"The most abundant mineral is the black iron oxide, which is not magnetic 

 except with the electromagnet, which was used to separate it from the remainder 

 of the powder. This iron oxide gives a reaction for titanium, and hence is rather 

 certainly ilmenite. Corundum, garnet, and feldspar are also present, and several 

 other minerals, as yet undetermined." 



The rocks which form nearly all the little island of Santo Aleixo are 

 rhyolites. Several years ago I submitted to Dr George H. Williams 

 specimens that I had collected on Santo Aleixo. He found them to be 

 rhyolites, and wrote as follows regarding them : 



"They are quartz orthoclase aggregates with almost no bisilicate constituents, 

 but their structures are varied. One of the specimens has a granular holocrys- 

 talline groundmass of quartz and feldspar, some chlorite, possibly representing 

 original hornblende or mica, and considerable blue tourmaline. The other Santo 

 Aleixo specimen is a fine granophyre. Porphyritic quartz and feldspar lie in a 

 holocrystalline groundmass which is filled with beautiful spherulites, showing the 

 black cross between crossed nicols. . . . There is no nephelene in any of these 

 rocks." 



Professor Derby has lately suggested in a private letter, dated July 

 29, 1901, the theory that the Santo Aleixo rocks "might prove to have 



