. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 98 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 val
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 98 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
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