QUATERNARY DEPOSITS 499 



Thence upstream for many miles the country maintains the same 

 character. The river trench averages probably 150 yards wide and 20 

 feet deep. One-third of it is occupied by the fine gravel bars and the 

 remainder by the stream, which is relatively shallow and flows with a 

 good current. The banks of variegated clays or clayey silts alternate 

 with equally high and steep banks, consisting of fine gravel and rusty 

 colored sand at the base and stratified brown sand above. The varie- 

 gated clays may be the back-swamp deposits and the sand banks the 

 material filling old channels or built up on the inner sides of the curves, 

 so that both classes, though rather strongly contrasted in appearance, 

 may represent the same general period of alluvial deposition. 



At the village of Ryapura there was a bar of quartz gravel 4 feet high 

 and 20 yards wide. The maximum diameter of the pebbles was 2 

 inches, though few exceeded 1 inch. Some chalcedony and brown chert 

 was noted. The gravel was rusty colored within 18 inches of the river 

 level, indicating the presence of considerable of an iron salt in the water 

 at the low stages. The bank behind the bar was 18 feet high above the 

 stream; no high ground was in sight from the top, but the tropical 

 vegetation prevented an extended view. 



Above a point probably about 100 miles from the mouth the river 

 trench is about 200 yards wide and the banks 15 to 18 feet above the 

 low-water stage of the stream. The gravel in the bars has become 

 coarser and in places has been cemented by limonite into a soft con- 

 glomerate. Gravel rises high in the banks and is overlain by brown 

 sand. The variegated silts are scarcely represented. Small creeks enter 

 the river from very narrow V-shaped trenches. 



At one place the river trench passes out of the old sand-filled channel 

 into the variegated silts, where it is noticeably narrowed and the stream 

 deep. Near the water level the bank has a nearly black layer that is 

 probably due to the accumulation of vegetable matter in the old back 

 swamp. Higher there are several dark gray layers of similar origin. 

 Dark bluish gray and greenish layers and lenses in the fine gravel and 

 sand layers of the old channels are common near the low-water level and 

 are probably due to local deoxidation of the reddish brown material. 

 The dark layers in the variegated silts are more regular and I think 

 were originally dark in color. After several miles in which the banks 

 are largely gravel, brown sand appears again and is finally succeeded 

 by the variegated silts a short distance below Saclin. 



At Saclin, which is said to be 115 miles from the mouth of the river, 

 we encountered the first ground higher than the Modern alluvial plain. 

 The village is situated on a bluff which rises 35 feet above the low- 



