TRACHY.TES OF THE PERNAMBUCO COAST 59 



Following the beach southward from Pernambuco, the Tertiary hills 

 that are exposed north and west of the city only reach the coast again 

 to the south near the village of Paiva. Here they are about one kilo- 

 meter to the southwest of the beach. The beds are horizontal, and are 

 composed of sands, clays, and gravels, and contain no fossils. 



TRACHYTES 



Two kilometers northwest of the point known as Pedras Pretas the 

 hills come quite down to the beach, but here the hills are of trachyte, 

 with a thin covering of Tertiary sedimentary beds capping them. Near 

 the cape Pedras Pretas the trachytes are quite bare, but over them are a 

 few waterworn quartz pebbles showing that the Tertiary beds have been 

 stripped away. 



On weathering the trachyte turns red, yellow, and purple. It has 

 been quarried at the point of land for making street paving blocks for 

 the city of Pernambuco, but the quarries are now no longer worked. 

 Specimens of these trachytes were collected and submitted to Mr H. W. 

 Turner, who kindly furnishes the following descriptions : 



" These rocks are typical trachytes. Macroscopically they are fine grained pur- 

 plish rocks with rather abundant phenocrysts of feldspar, some of which attain a 

 length of 6 centimeters. 



"Microscopically the trachyte is composed of idiomorphic feldspars in a fine 

 grained groundmass of feldspar laths, with indistinct boundaries, which show a 

 tendency to arrange themselves in parallel lines, which curve about the ends of 

 the feldspar phenocrysts, thus exhibiting a typical trachyte texture. Some of the 

 phenocrysts, as well as the larger part of the microlites of the groundmass, extin- 

 guish sensibly parallel to their direction of elongation, and have an index of re- 

 fraction less than that of the balsam. They are thus orthoclase. A few of the 

 phenocrysts are microperthite, showing minute lamellae, presumably of albite and 

 orthoclase intergrown. These lamellae extinguish at different angles. A few feld- 

 spar laths of the groundmass show albite twinning and extinguish at slight angles. 

 These are probably oligoclase. There are some elongated grains of quartz without 

 crystallographic boundaries present, and these appear to have formed after the 

 feldspars, but, nevertheless, to be original. The microlites show no tendency to 

 arrange themselves around these quartz grains, and in some cases the ends of the 

 feldspars are enclosed in the quartz. A few flattened, nearly rectangular prisms 

 with high relief and brilliant interference colors extinguishing parallel to the prism 

 are probably zircon. There are also rather numerous opaque grains of iron oxide, 

 probably magnetite. The section is obscured by a dust of particles, some of them 

 nearly opaque, but where thin are translucent with red-brown color. These are 

 perhaps limonite formed from the alteration of magnetite, for the magnetite grains 

 show a thin rim of similar material. These reddish grains give the purple color 

 to the rock. A little carbonate is also noted.". 



On the Pedras Pretas point are several blocks of the trachyte, beauti- 

 fully pitted by sea urchins. These blocks are now so far above mean 



