142 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
face that they are nearly perpendicular. The sketch below was made 
from a point further south, and looking due west toward the town. 
At Porto Seguro the flat-bottomed nature of the valley is more striking 
than at Santa Cruz. The hills ате the same Tertiary sediments, with 
valleys cut down in them. At the north edge of the upper city one of 
these valleys appears not to have been quite deep enough to let the sea 
into it, but the valley of the Rio Cachoeira being larger, there was here 
formerly a long narrow estuary that has become silted up and turned 
into dry land. 
The view from the top of the south edge of the plateau near the upper 
city at Porto Seguro is an impressive one. The slope of the bluffs is 
close to 45°, the valley is broad, almost perfectly flat, and less than a 
metre above high tide-level. The sketch on p. 98 will give an idea of 
the more prominent features of the region. 
Some interesting examples are visible on the cliffs exposed north of 
Prado, State of Bahia. The following sketch shows the barreiras, or red 
clay bluffs, just south of Comoxatiba in the vicinity of the monazite sand 
deposits. 
Fic. 80, Sketch of the truncated hills north of Prado, as seen from the ocean. 
The striking feature of this view is that some of the little valleys stand 
high above the sea-level, while others appear to have sunk quite below it. 
The history appears to have been an elevation after the deposition of 
the sediments ; during this elevation the drainage cut the valleys. ‘Then 
followed a depression, and the lower valleys went beneath the sea, while 
the higher portions were not affected. Marine erosion has truncated 
and exposed many of these valleys in the bluffs. 
That the estuary valleys are sometimes still partly open or not 
quite filled up is due to the scouring action of the tides. For estuaries 
with streams entering their upper ends have to discharge at ebb tide 
more water than they receive during flood tide, hence the outflowing 
current must be more rapid than the inflowing one. They can only fill 
up from the upper or landward ends, and by the combined action of 
mangroves and of silts brought down by the drainage. The seashore 
silts cannot reach во far up streams. 
Should it be suggested that these flat lands might be produced by an 
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