GRANITES AND DIOKITES OF CABO SANTO AGOSTINHO 61 



On the south side of the cape the granites are of two kinds — coarse 

 granites and granite-porphyries. Through the porphyries is an altered 

 dike of diorite, running about parallel with the hill, between the old 

 fort at the bar and halfway to the village of Suape. The rock on both 

 sides of the diorite is granite-porphyry, and small dikes of the porphyry 

 penetrate the larger dike of dark green diorite. The old fort on the 

 point of the cape near the Barra do Suape stands on the porphyry, but 

 farther north the rock is a granite. These three rocks from Cabo Santo 

 Agostinho have also been described by Mr Turner as follows : 



"The' granite from cape Santo Agostinho is a coarse rock composed of ortho- 

 clase, inicroperthite, and quartz, with frequent wedges of a strongly pleochroic 

 green-blue amphibole between the other constituents. 



"This amphibole is in the form of longitudinally striated prisms, which are 

 black as seen with a hand lens. The pleochroism is strongest (dense blue) where 

 the cleavage lines are parallel to the horizontal cross-hair. The extinction was 

 not determined on account of the dense color. 



"A fragment of this amphibole was treated with hydrofluosilicic acid, there re- 

 sulting little hexagonal sodium fluosilicates, a few octahedral anistropic crystals 

 of undetermined nature, and some thorn-like anistropic forms radiating from a 

 center much resembling calcium fluosilicates. The presence of sodium, together 

 with the character of the pleochroism, suggests that this amphibole is allied to 

 riebeckite. Calcium is present in some analyses of riebeckite.* 



" The granite forming a dike in the diorite is, macroscopically, a light colored 

 coarse rock, composed of light-buff feldspar, quartz, and dark-greenish material. 



"Microscopically, it is composed of orthoclase, inicroperthite, and quartz, with 

 a little reddish-brown strongly pleochroic biotite and a strongly pleochroic green- 

 blue amphibole, with marked cleavage resembling riebeckite and similar to the 

 amphibole described under the preceding specimen. 



" The granite porphyries from cape Santo Agostinho are macroscopically light- 

 gray fine grained granolites, showing porphyritic quartzes up to 1J millimeters 

 in diameter and porphyritic pinkish feldspars up to 2 millimeters in diameter. 



"Microscopically, the rock contains numerous phenocrysts of turbid feldspars, 

 often in simple twins, and squarish and hexagonal, sharply idiomorphic, quartz 

 phenocrysts in a microgranular groundmass of quartz and feldspar. The feldspar, 

 both in the phenocrysts and in the groundmass, is largely inicroperthite, but 

 orthoclase is also present. There are occasional small opaque grains of metallic 

 iron oxide, probably magnetite, and grains and minute prisms with high relief, 

 showing strong cleavage and bright interference colors extinguishing parallel to 

 the prism. 



"In one specimen of the granite porphyry there are very abundant minute 

 rectangular pleochroic crystals showing a single cleavage parallel to the sides of 

 the rectangle. The pleochroism is reddish brown when the vertical cross-hair is 

 parallel to the cleavage, and nearly black at right angles to this direction. 



"The diorite from cape Santo Agostinho is macroscopically dark and fine 

 grained, microscopically a cataclastic igneous rock, showing crushed feldspars and 



♦Dana : System of Mineralogy. 



