438 JENKINS— GEOLOGY OF THE [May 29, 



In the interior the rocks are crystalhne : granites, gneisses, schists, 

 shales, quartzites, and various forms of intrusive dikes. At Maca- 

 hyba an engineer, who had been far back into the country, reported 

 marble at a distance of about one hundred and* sixty kilometers from 

 the coast. This marble was said to have been like that found near 

 Quixada in Ceara, at the same elevation and relative location. The 

 granitic rocks wihich are nearest the coast, in the region which this 

 paper deals, occur at Alacahyba, for here the river cuts deeply 

 through the sedimentaries and exposes the older series. 



Overlying the crystalline series are beds of fossiliferous lime- 

 stone, lying almost horizontally, or dipping about 5° southeast, 

 towards the coast, in layers of a few inches to a foot or more in 

 thickness. This series is not well exposed. The principal localities 

 for the exposures are at the railway cuts and at the quarries made 

 in the limestones where they come to the surface along the sides 

 of the valleys. Following along the contact of the granites with the 

 sedimentaries one may find occasional indications of limestone on 

 the surface. There seems to be a belt of this limestone, left from 

 the great erosion the country has undergone, about ten kilometers in 

 width. A few pebbles of limestone were found on the surface not 

 far from Taipu, thus indicating a greater extent of the limestones 

 in previous ages. 



Unconformably overlying the limestones occurs anunfossiliferous 

 iron-sandstone and clay series of generally unconsolidated material. 

 The beds of this series are hard to distinguish, but they also seem to 

 lie horizontal or to dip gently towards the coast. They extend in 

 a belt about thirty kilometers in width along the coast. They are 

 best exposed as sea-cliffs. 



The wide river valleys contain a deep deposit of alluvium. This 

 is an important factor in the geology of the country for these deposits 

 are found to contain marine shells, showing the valleys were once 

 filled with water from the sea, and later completely silted up. This 

 process of silting-up is at present at work in some rivers as Rio 

 Jundiahy and Rio Potengy, where it occurs about twenty-five kilo- 

 meters inland. It is a mark of a submerged coast. 



Over all the other formations a vast amount of sand is at present 



