GEOLOGICAL CONCLUSIONS. 149 
by the position of a growing reef; and that therefore there would be un- 
occupied intervals or channels, as above alluded to, between the seve- 
ral reefs of a reef-ground. 
The bearing of these facts upon the character and origin of ancient 
limestones, and the formation of channels, or valleys in such rocks, is 
apparent without particular explanation. 
III. ‘The occurrence of coral sand forming the exposed beaches, 
while the finer coral mud exists on the shores of the smaller lagoons, 
or at the bottom of the larger, affords an interesting illustration of the 
result produced by a triturating sea, as compared with that from more 
gently agitated waters. ‘The seashore waves give rise to sand or 
pebbles; while the gentle undulations or ripplings of inland waters 
produce mud by their finer trituration, (p. 108.) 
IV. The beach conglomerate or sandrock, and the drift sandrock 
are examples of stratified deposits forming and consolidating along 
shores. The former has a nearly constant dip of a few degrees, not 
exceeding eight, towards the water. The latter is more finely lami- 
nated, less firmly cemented, and dips whichever way the accumulating 
sandhill sloped, the layers being the successive sheets of sand which 
were drifted over it, during the accumulation, (p. 45.) 
V. The almost total absence of fossils from many parts of the coral 
reef-rock, and generally from the shore sandrocks, is one of the most 
striking facts here exemplified. These rocks are formed in the midst 
of life, and out of the enduring remains of animals; yet fossils, as 
shown at Metia and other elevated reefs, are often rare. 
This absence of organic remains characterizes almost invariably 
the drift sandrock. On Oahu, where this rock forms hills thirty or 
forty feet in height above the reef-rock, not a fossil nor a fragment of 
one was distinguished by us, neither of shells nor corals. ‘This fact 
had been previously remarked by some of the intelligent residents, 
and it was a matter of dispute whether one or two shells had not been 
found. These formations are but a few rods from waters prolific with 
the productions of the sea, and were made from them. 
An explanation of this peculiarity, is obvious on the principle 
already discussed —the action of a triturating sea. Everything 
washed towards the shores, is ground down by this action and reduced 
to sand; and a large part of the sand is worn out and carried off by 
the sea ; or, being thrown up by the reef, is blown inward by the winds. 
It is a natural inference from these facts, that the non-fossiliferous 
sandstones of our continents are no good evidence of the absence or 
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