THE VEGETATION OF THE VIRGIN-FOREST. 
109 
I 
As every zono of geologic formation in tlie extensive valley adds 
its tribute, these banlcs ai’o a hind of mineralogical collection, which, 
shows sample.s of all the rocks on the river-banks ; with the ex- 
ception, perhaps, of light pnmiee-stono, the produce of the volcanoes 
of the Andes, which drifts down stream in lai’ge pieces, and is highly 
prized by the Tapuia population (on the lower course) for sharpening 
and cleaning their weapons and tools. Even Avhen not picked up 
by hunter or tisher, it is not lost. It will be arrested by some 
snag or projection of the shore, it will so get embedded in tlie 
newly-forming sediment, and thousands of years hence its silicic acid 
will afford the necessary material for 
the hard glassy bai'k of a bambusacca, 
or the sharp edge of a reed. When 
the currents are not strong enough to 
move the larger banks, they at least 
carry sand and earth with them, and 
dc'posit them as shoals or new alluvion 
at loss exposed spots.* Itut there is 
no stability in the liquid element, 
with its jieriodic lise and fall, and 
the restless working of the busy waves. 
l)i\ ert('d b}- the obstructing shoal, the 
river cats away the banks originally 
formed in the sea basin ; and the 
sharper the bend the quicker tin- 
demolition. Then begins to form a 
serpentine, whose vagrant coursci gets 
more and more punnounced by the 
concave bank breaking doAvn and forming new deposits on the conve.x 
one ; until, at last, an extra(3rdiuaiy flood breaks through the narrow 
isthmus and opens up a stinighter bed for the river, which soon 
resumes its playful operations afi-esh.t The convex bank, therefore, 
* Tho laws of these movements, the sedimoiits, &o., are of course the same in 
aU climes ; but in old Europe all tlie larger rivers we so controlled, regulated, and 
canalized, narrowed by dj'kes and other restraining apiiliauces, that this restless 
power, the wonderful jxtrpetxmm mobile of a lai’go river, must be entirely imperceptible, 
at least to non -professional observers. 
f M'o found places on tho Mamore where three .several bods of dilfercnt iieriods 
