608 RIO NEGRO, PATAGONTA., 
occasionally the banded colours of agate, or the bright ruby or flesh 
tints of the carnelian. Small pebbles of granite were rarely seen. 
The porphyry has a variety of colours, among which purple, red, and 
green were the most common. ‘The width of the beaches varies from 
two hundred to five hundred feet, and the inclination is but one to 
three degrees. There are large shoal flats in the harbour of Rio 
Negro, which are increasing annually and undergoing frequent 
changes. We were informed that a northern channel was open five 
years since, which is now too shallow for navigation. 
The sea-shore bluffs and the banks of the river up the stream ex- 
hibit sections of the underlying material of the pampas. The layers 
there exposed to view consist of sandy or clayey material, and are not 
at all indurated, beyond what might have resulted from pressure and 
such slight depositions of mineral matters from solution as may happen 
from ordinary cold waters. ‘The lower layer at the South Barranca 
is very argillaceous, soft and friable, and has a dirt-brown colour. It 
extends out as a flat bank, a few hundred feet wide, at the base of 
the bluff, and is covered with water at high tide. It has been eroded 
by the action of the water that breaks over it during the rise and fall 
of the tides, and then runs off in rills, so that it is now a collection of 
small islets, which may be passed over by stepping or jumping from 
one to another. The surface of this layer is mere mud to a depth 
of an inch, owing to the washing of the waters, which are thus restor- 
ing it to its original condition. This bed appears to be protected in 
the same manner as the shore platform of rock in New Zealand and 
New Holland (pp. 442, 532). 
The arenaceous layers, above this lowermost, are of various colours. 
Some are grayish-yellow, others purplish, and others are nearly white. 
The bluish-purple colour which characterizes one broad layer, appears 
to arise from the dissemination of particles of magnetic iron ore, which 
mingle their bluish-black colour with the colours of the sand. Among 
the upper layers there is a yellowish or grayish-white compact rock, 
containing dendrites of manganese, and rolled fragments of the same 
are scattered over the beach. 
The lines of deposition in these layers are generally very distinct. 
In one of them, many of the lamine are scarcely an eighth of an inch 
thick. Moreover they are much curved, and in many places not con- 
formable, precisely as we have described the sand-hills of Oahu, and 
the Sydney sandstone of New Holland. The figures given in our 
illustrations of the Oahu rock, (p. 255,) exhibit, with sufficient accu- 
