98 BULLETIN: MUSEUM OF COMPARATIVE 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- 
A B 
Fic. 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. 888) has described pot-holes often 
of great size and containing water, in the gneisses to the east of the 
Serra do Esperanco 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 
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