Waring: Geology of Northeastern Brazil. 189 



Well-bedded, gray, crystalline limestone forms a dam site at Barra 

 do Mainoso, forty-seven kilometers from Acary. The principal expo- 

 sure strikes S. 20° W., with a dip of forty degrees eastward. The 

 eastern limit of the limestone is marked by a ledge in the channel of 

 the stream, which has the same strike and dip as the main ledge, and 

 by a ledge of gneiss one hundred and fifteen meters farther to the east 

 in the channel, having conformable structure. Pebbles of iron 

 oxide, some with prismoidal faces, are scattered over the surface of the 

 gneiss for several hundred meters eastward from the limestone. To 

 the west the limestone extends for about two kilometers, where gneiss 

 reappears and becomes more granitic toward Caico. Several large 

 ledges which seemed to be of white quartz, but may be of pegmatite, 

 cut the granitic gneiss northeast of Caico. One ledge, striking south, 

 forms a dam site on a small stream five kilometers north of the city. 

 Northwest from Caico the granitic gneiss contains considerable quartz 

 along its foliation planes, which swing in strike from S. 20° E. through 

 south to west. Ten or twelve kilometers northwest of Caico there is a 

 small granite hill; granite is also exposed in a stream-channel at 

 Jardim de Piranhas; and Brejo da Cruz is at the western base of a 

 prominent granite hill with steep, smooth, spalled-oft' slopes, the long 

 axis of which trends N. 40° W. Thence northward the surface is 

 granitic rather than gneissic. Two shallow intermittent ponds in this 

 locality are worthy of mention, because even intermittent ponds are 

 rare in the region and were observed only in granitic not in gneissic 

 areas. The most notable occurrence of such ponds is two hundred 

 kilometers to the northwest, between Quixeramabim and Prudento de 

 Moraes. If this observation is correct, it may be accounted for by 

 the greater difiiculty of underground drainage down through the 

 granite, than down along the foliation planes of the gneiss. To and 

 beyond Belem the surface of the plateau continues mainly granitic, 

 though with some gneissic phases. Numerous granitic hills rise 

 above the mean level. Patu is at the northwestern base of such a 

 large hill. North from Patu there is a wide plateau of granite, con- 

 taining feldspars from one to two centimeters in length. The illus- 

 tration given on Plate YII, fig. 2, shows some of the granite hills a few 

 kilometers north of Patu. 



About halfway between Patu and Apody the rock becomes more 

 banded and gneissic, and twenty-five kilometers southeast of Apody 

 the road crosses a ledge of crumpled, siliceous-calcareous schist a few 



