ROCKS OF RIO FORMOSO AND PEDRO DO PORTO 63 



some connection as an acid phase with the Fernando de Noronha 

 eruptive epoch, and that the same magma might give a granite in the 

 neighborhood." 



RIO FORMOSO ROCKS 



The Tertiary sediments are exposed in all the prominent hills and 

 occasionally on the streams or estuaries at low tide south of Santo Aleixo 

 as far as the mouth of Rio Formoso. At Rio Formoso the granites are 

 exposed on the south side of the river about 3 kilometers from the sea, 

 and from that point up stream as far as the town of Rio Formoso. There 

 is therefore only a narrow belt of the Tertiary at this place. 



The hills near the coast are all Tertiary and isolated, but between 

 them are occasional exposures of soft sediments that appear to be of 

 later Tertiary age. Along the shore, between the mouth of Rio Formoso 

 and Praia da Gamella is an interesting exposure of these later Tertiary 

 rocks. The beds are horizontal, and are being rapidly attacked by the 

 waves, marked changes having taken place between January, 1876, when 

 the locality was first examined, and July, 1899. The lowest bed exposed 

 is soft white sandstone, 2 meters thick. On top of this is a bed of soft 

 black sandstone, the two being separated by about 2 centimeters of yellow 

 cla}'. On top of these two beds is beautiful white quartz sand from 3 to 

 6 meters thick. The bed in place is 3 meters thick, and in some places 

 it has been blown over and another 3 meters heaped on top. This sand 

 appears to be available for the manufacture of glass, and there is an 

 abundance of it. 



These Tertiary beds are exposed from Gamella nearly to the hill on 

 which stands the church of Nossa Senhora de Santa Anna and against 

 which the beds end. The hill of Nossa Senhora de Santa Anna is of 

 Tertiary rocks also, but these beds belong to the older series. 



GRANITE OF PEDRA DO PORTO 



Northwest of the village of Tamandare the horizontal colored Tertiary 

 is exposed in the hills at two places. About 6 kilometers south of 

 Tamandare the hills come down to the coast and the granites are beauti- 

 fully exposed on the beach at a point known as Pedra do Porto. The 

 following is Mr H. W. Turner's description of the granite found at the 

 Pedra do Porto : 



"The biotite-granite from Pedra do Porto is macroscopically a light gray coarse 

 rock, with large pink feldspar phenocrysts in a coarse groundmass of quartz, feld- 

 spar, and biotite. 



"Microscopically composed of orthoclase, microperthite, microcline, micro- 

 pegmatite, oligoclase, quartz, and biotite. As accessories there are present iron- 

 oxide, apatite, titanite, and a xenotime-like mineral." 



