188 Annals of the Carnegie Museum. 



cai^pcd tableland to the southwest. The principal pegmatite dikes 

 strike S. 30° \\"., parallel with the valley of the stream, but minor ones 

 branch off in various directions. Eight kilometers north of Periquitos 

 the pegmatite carries much muscovite and black tourmaline. Farther 

 northward the pegmatite is finer in texture and associated with mica- 

 gneiss. On the east side of the valley the gneiss dips westward at 

 forty to fifty degrees, but the dip seems to be less steep on the west side. 



One kilometer south of Parellas dark mica-gneiss strikes S. 50° W., 

 and nearby a prominent pegmatite ledge trends S. 35° W. Northwest 

 from Parellas the gneiss continues with strike S. 50° W., and twelve 

 kilometers from this village pegmatite again becomes common, many 

 dikes cutting the gneiss with fairly constant southwest strike and 

 vertical dip. 



Three kilometers southwest of Jardim de Serido agranitic or haplitic 

 dike cuts southward across a river-channel, and tw^o and one-half 

 kilometers southeast of the town a pegmatite dike trends southwest 

 across another stream-channel, as shown on Plate VI, fig. 2. About 

 one hundred and seventy-five meters downstream from it a waterfall 

 of three meters is formed by another granitic ledge, but the main 

 country-rock is gneiss, with general southwest strike. In this are 

 intruded areas of granite, which form isolated, half-barren hills. 

 Crystals of muscovite mica as much as five or six centimeters in dimen- 

 sions are plentiful in the pegmatite of the locality, but they are flecked 

 with iron. Garnets up to one centimeter in diameter are also com- 

 mon, and some graphic granite was seen. To the northeast many 

 pegmatite dikes cut the gneiss in all directions. About half-way 

 between Jardim de Serido and Acary the gneiss becomes more granitic 

 in texture, and thence northward pegmatite is less common. At 

 Acary the rock is granitic rather than gneissic, and at the Gargalheira 

 dam site, a few kilometers to the northeast, it is a hard, fresh, por- 

 phyritic material. In the narrow stream-channel the boulders and 

 bedrock are deeply w^orn into pits and potholes, as shown on Plate \TI, 

 fig. I. For six or eight kilometers westward from Acary many peg- 

 matite dikes cut the granite and gneiss, but farther west pegmatite 

 is less common and decomposed gneiss becomes the predominant 

 phase of the surface-rock. Near the junction of the Rios Acauan 

 and Serido, thirty kilometers southwest of Acary. the strike of the 

 gneiss swings from the prevailing west or southwest to about S. 20° W., 

 and pegmatite becomes scarce. 



1 



