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 are the same Tertiaiy 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 volley of the Rio Cachoeira being larger, there was here 

 formerly a long narrow estuary that has become silted up and tui'ned 

 into dry laud. 



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 tidedevel. 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 harreiras, or red 

 clay bluffs, just south of Comoxatiba in the vicinity of the monazite sand 

 deposits. 



Fig. 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 seadevel, 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 vallej-s 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 so far up streams. 



Should it be suggested that these flat lands might be produced by an 



