108 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
to be as follows: — The streams and wet-weather wash concentrate’ 
pebbles in their 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 débris in the stream channels and erosion proceeds more rapidly 
Fic. 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 sandstone beds. 
along them so that they become depressions between the old deposits. 
Concentration of coarse débris 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 
Tamandud 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 were signalized in south Brazil by a heavier rainfall than 
that now prevailing it is probable that the older gravels represent one: 
= sebbbi i> @if 
