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fox a time. Some rock masses, after having had holes worn on one 
face, and been subsequently detached from their position and in- 
verted, and again eroded on another face, present the singular ap- 
pearance of having cavities on upper and under surfaces. In the 
formation and enlargement of the basins, Mr. Newbold is of opi- 
nion, that the erosion is the work of water, assisted only by the 
effects of the atmosphere upon the rock during the dry season ; and 
he thinks that these two agents are fully adequate to make the ca- 
vities with the aid of a natural decomposition of the strata or the 
attrition of blocks and pebbles rolled along by the current, though 
he admits their cooperation to a certain extent. The manner in 
which he considers the basins are formed is as follows. The water 
having worn a hole, however small, in the rock, flows into it in a 
circling eddy, and thereby enlarges the sides and bottom in a greater 
ratio than the orifice. ‘This mode of operation, he says, may be 
demonstrated by throwing a fragment of cork into the current before 
it enters the cavity, and by then watching the gyrations of the cork 
till it escapes over the lip of the basin. During this experiment it 
will be seen, that the centre of the bottom is but little acted upon, 
and that the projections before noticed are consequently left. That 
these cones do not rise to the level of the orifice, Mr. Newbold says, 
is accounted for by the action of the water in the shallow cavities 
being more equally distributed over the whole superficies of the in- 
terior, and from the formation of the projections not commencing 
till the basins have been deepened. 
The cavities are mostly free from sediment, but some contain 
pebbles and sand disposed in a horizontal bed at the bottom, un- 
disturbed by the rotatory motion of the water. In all cases in which 
Mr. Newbold noticed earthy matter carried into the basins by the 
current, the weightier pebbles sunk immediately, and either re- 
mained stationary or were but slightly moved ; and the heavier par- 
ticles of sand also sunk after making one or two whirls round the 
interior of the basin, while the mud and other light materials passed 
with the upper current over the lip at the opposite side. 
During the dry seasons, when the contents of the basins are gra- 
dually evaporated, the carbonic acid contained in the water, acts, 
Mr. Newbold says, upon the rock which frequently possesses a 
temperature of 120°, and softening the interior of the cavity, pre- 
pares it for additional erosive effects by the river during the next 
monsoon. 
Besides the river-basins, the author alludes ¢o similar but smaller 
cavities on the surface of rocks at considerable elevations above the 
drainage level of the country, and which result from the action of 
springs or rain-water overflowing from receptacles where it had 
collected ; also to other hollows not referable to similar agency, on 
the summits of table-lands and isolated mountain-peaks, where no 
springs or collections of rain-water have been known to exist. Ca- 
vities of this description the author has observed on the summit of 
limestone mountains in Greece, Sicily, the south of Spain, the op- 
