420 THE CKETACEOUS AND TERTIARY GEOLOGY 



and there upon exposure, forming quartzite blocks. As the surrounding portions of 

 the beds weather away, these blocks are usually left lying loose on the surface where 

 they exfoliate somewhat under the same influences that hardened them. That this 

 metamorphism is produced by weathering scarcely admits of doubt. The beds 

 which are metamorphosed are high ;ip in the tertiary series, the underlying beds being 

 made up of soft horizontally stratified beds of clays and sands. It is quite evident 

 that these rocks have not been subjected to any unusual lateral pressure, such as that 

 which so often produces metamorphism. The jiressure from above is and always has 

 been less than that upon the underlying soft beds of sand, so that its metamorphism 

 cannot be attributed to pressure from above. The strongest evidence that this meta- 

 morphism is a process of weathering is found in the condition of some of the par- 

 tially metamorphosed masses of sandstone. The writer has seen at three separate 

 exposures large masses of this glassy qaartzite protruding from banks of soft sand- 

 stone, which, upon being broken ofi' two or three feet beneath the face of the expo- 

 sure, showed the uncovered end of the block to be nothing more than sand in piocess 

 of hardening, the mass becoming harder and harder toward the exposed surface. 

 When these blocks are entirely separated from their surroundings and lie fully exposed 

 to sunshine and rain, they become as hard as glass and have a similar conchoidal 

 fracture. Heated as they usually are by the direct I'ays of the sun during the day 

 and cooled by radiation at night or by cool rains, their angles exfoliate until they are 

 almost })erfectl3^ lound. Prof, Hartt at one time thought that these boulders were 

 brought to this region by glacial action. Although these quartzites arc tertiary, 

 there are occasional quartzitic beds in the cretaceous also, yet there is but little 

 danger of confounding them ; those of the tertiary being barren of fossils, while those 

 from the cretaceous usually contain a few fossils. The hard cretaceous sandstones in 

 the Riacho de Aroeira approximate in general appearance to the tertiary quartzites. 

 It would not be safe, however, to assign all the non-fossiliferous quartzites of this 

 region to the tertiary, for some of the cretaceous sandstones have been sufiiciently 

 porous to allow percolating waters to dissolve out the fossils, and subsequently they 

 have been changed to quartzites. I find in my notes the opinion expressed that a 

 certain bed of quartzite more or less buried beneath black cretaceous limestone soils, 

 is cretaceous, and uncovered by the decay of overlying beds of limestone. 



Tertiary quartzites were observed in the Sergipe basin at the following localities : 



]. Sitio da Ribeira, on the Rio Sergipe as one ascends the stream from Aracajii 



towards Piutos, on the north-west side, opposite Andorinhas quarries. These blocks 



have evidently been derived fioni beds at a higher elevation and have been left here 



by denudation. 



