170 BRANNER— THE FLUTING AND PITTING [April 4, 



The Origin of the Caldrons. 



The suggestion of Hartt quoted on a preceding page in regard to 

 the glacial origin of caldrons or pot-holes in the interior of Bahia 

 must be set aside as quite out of the question, either for the ones in 

 Ceara or for those in Bahia.* There is absolutely no evidence of 

 Pleistocene glaciation in Brazil.^ The pot-holes mentioned by Hartt 

 and Allen in the interior of Bahia, in every case with which I am 

 acquainted, have been cleaned out by man for the purpose of using 

 them for water storage in times of drouth. They have but little in 

 common with the caldrons referred to in this paper. 



The Quixada pits are in process of development, and they are 

 evidently therefore to be explained by agencies now in operation. 

 xA.side from the openly exposed bare surface of a coarsely crystalline 

 homogeneous rock, the factors that enter into the problem appear 

 to be : The slope of the surface, the localization of incipient depres- 

 sions, disintegration and chemical alteration of the rock forming 

 minerals by the action of rainwater aided by organic and other 

 acids, the concentration of rainfall, and the mechanical floating away 

 of the disintegrated and finely divided minerals. 



It seems probable that the original location of the incipient cal- 

 drons is purely accidental. Once begun, however, the standing water 

 attacks the constituent minerals of the rock in the bottom of the pit. 

 It is quite noticeable that in the great majority of the caldrons, per- 

 haps in all of them, the walls are somewhat overhanging. This over- 

 hanging is due to the fact that once a pit is started, the water stand- 

 ing in it tends to moisten and affect a widening surface. 



As soon as a depression is deep enough to retain some of the 

 rainwater, plants begin to grow in it, and insects are drowned in it, 

 and as these things decompose the organic acids thus produced en- 

 able the water to attack the minerals more readily. 



The concentration of the rainfall of the region causes the pits to 

 be filled with water for only a part of the time. The rest of the time 

 the pits are dry or they are only moist on the bottom. 



It has been suggested that deflation may be an agency in the for- 



*Hartt's "Geology and Physical Geography of Brazil," pp. 314-315- 

 ' " The Supposed Glaciation of Brazil," Jour. Geol, I., 753-77^- 



