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BULLETIX: MUSEUM OF COMPARATIVE ZOOLOGY, 



to be as follows : — The streams and wet-weather wash concentrate 

 pebbles in then* beds leaving the interstream swells or rounded ridges 

 comparatively free from coarse material. The finer decomposed ma- 

 terial of the interstream areas becomes more readily eroded than the 

 coarse debris in the stream channels and erosion proceeds more rapidly 



Fig. .30. — Showing supposed stages, A. B. C, in the concentration of gravels 

 in creases and the deposits on ridges between creases on the dip-slope of 

 pebbly sand.stone beds. 



along them so that they become depressions between the old deposits. 

 Concentration of coarse debris begins again in the now new well- 

 defined creases which carry off the rainfall. So far as my observations 

 go there have been but two cycles of such gravel accumulation, an 

 ancient one and that now actually taking place. If the process 

 depends solely upon the relative resistance to erosion of the gravel- 

 bearing creases and the gravel-free ridges between streams the change 

 may well be automatic under a constant rainfall during the period 

 of alteration. After one such shift the interstream areas become 

 partly gravel -capped and an equilibrium is established which at first 

 did not exist. Thus the dual character of the phenomenon in Brazil 

 may be due to this limitation inherent in the nature of the process, 

 even with a variable rainfall. 



In the case of the deposits of this nature on the Serrinha near 

 Tamandua in Parana the ancient gravels occur in abundance and 

 appear to exceed in thickness those of the present wet-weathered 

 channels. That these older gravels in this climate are as old as the 

 Tertiary period seems to me improbable since under the conditions 

 of exposure to weathering they must have broken down. At best they 

 might be Pliocene, but if it is admitted that the glacial epochs of the 

 Pleistocene Avere signalized in south Brazil by a heavier rainfall than 

 that now prevailing it is probable that the older gravels represent one 



