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bulletin: museum of compakative zoology. 



The exposures of trap on the rounded ridges which separate the 

 basins are usually crusted with superficially segregated oxides of iron, 

 and loose weathered blocks are not infrequently seen in positions to 

 indicate the almost complete absence, for a long geological period in 

 the past, of any transporting agency such as sliding snow, ice, or run- 



-^Tffrffrrr 



A B 



Fig. 27. — Basins of decomposition on the Triassic trap plateau. A, cross- 

 section of basin overhanging a stream valley. B, contour map of lakelet 

 converted into a swamp. Santa Catharina. 



ning water. There is thus no reason for supposing that the basins are 

 due to other causes than deep secular decay and the slow wasting away 

 of the rock under a moderate rainfall. That these weathered basins 

 are of great antiquity is obvious from the consideration of the mode 

 of origin which thus may be ascribed to them. There is no clear local 

 indication of the geological date of beginning of the basins. Inasmuch 

 as they abound on the surface of the Corisco lava-flow above described 

 they, in this instance, are more recent than the erosion of the overlying 

 sheets of trap. I saw nothing in them by which to distinguish 

 Pleistocene from Tertiary processes unless it be the deposits of clay 

 which would argue for probably a Tertiary date as the time of begin- 

 ning of the corrosion, but they may be early rather than late Tertiary. 



Such solution-basins are not limited to the trap plateau but are 

 to be seen here and there on the Permian area in Sao Paulo where 

 springs find their way to the surface. 



Mr. T. A. Allen (Derby, 1906, p. 388) has described pot-holes often 

 of great size and containing water, in the gneisses to the east of the 

 Serra do Esperanto on the plains of Bahia. He regarded these pits 

 as due to a peculiarly localized action of disintegration. 



Professor Pumpelly (1879, p. 136) has called attention to the manner 

 in which deep secular weathering followed by a period of active erosion 

 as by ice would result in the production of a topography quite unlike 

 that of normal land sculpture by streams. He notes that " as masses 



