RIVER-DEPOSIT OF THE AMAZON. 767 
sections. The deposit here consisted of grey and white sand beds 
for almost the full height of the cliff; upon them rested white clay 
with red loam on top. In the centre of the bay rose a small, narrow 
peninsula of whitish-grey sand, forming a thin peaked ridge of about 
one half the height of the cliff, and joining with the base of the cliff 
in the middle of the back of the scoop. In this sand were pellets of 
blue clay, and in some parts trunks of large trees in a semicarbonized 
state, showing that they had been imbedded for a long period. The 
washing-out of this great mass of material, of 600 feet in diameter 
by 80 feet in height, was performed mainly by the river, the current 
of which made an eddy at its edge at the foot of the cliff, and thus 
cut rapidly into the soft sand of which the latter is composed. The 
force thus exerted was probably assisted by springs of water which 
issued through the base of the sand above the bluish clay. 
Guara Barreira rises to a height of only 10 feet above flood-mark, 
and is composed of the following beds :— 
ft. in. 
0 5 Brown loam. 
11 0 Red arenaceous clay, passing into mottled red and grey clay. 
12 0 Banded red and grey clay-beds. 
30 Yellow sand. 
20 0 Light-yellow sand. 
Here the river’s level at the time was 6 feet above the lowest point 
it ever attains, and was by measurement 363 feet below the water- 
mark of the highest point it reaches in the rainy season. There is 
a constant fluctuation in the level of the river day by day ; for, even 
when subsiding gradually, it will sometimes rise a few feet, and then 
as suddenly fall again. This is owing to heavy rains in the Andes, 
or at the source of some of its large tributaries, producing a sudden 
accession of water to the main river. 
At Fonte Boa Barreiras, 60 feet in height of which was exposed, 
the following sections, taken not far apart, show a considerable 
difference in the composition of the beds of the deposit over a small 
area :— 
feet. 
( 3 Red loam. 
I 37 Red sand. 
*4 8 Yellowish sand. 
| 5 Red, pink, yellow, whitish, and variegated plastic clay. 
7 Yellow sand. 
\ 
( Red loam. 
30 4 Mottled clay. 
| Grey and red-banded plastic clay. 
Red speckled sand. 
Grey sand. 
| Yellow sand. 
A section at Jutahy Barreiras gave the followin g :— 
ft. 
5 Yellow loam. 
10 Red loam. 
1 Purple clay. 
14 Reddish-pink sand. 
30 Yellow sand. 
3r2 
