392 
NATURE 
[FEBRUARY 25, 1897 
it uniform subsidence of mounds, of identical height, over 
a great area? Is it building up of mounds to an identical 
distance below the surface? I cannot think that either 
can account for the conditions. I would venture to 
suggest the cutting down of volcanic islands by the 
action of the sea, and that this operation has a far 
greater share in furnishing coral foundations than has 
generally been admitted. 
The operation has not been overlooked. Mr. J. 
Murray says: “Volcanic mountains... like Graham 
Island, might be wholly swept away, and only a bank 
with a few fathoms of water over it be left on the spot. 
In this way numerous foundations may have been pre- 
pared for . . even atolls” (Proc. Roy. Soc. Edin., 
vol. x. p. 507). Sir A. Geikie, in referring to Murray’s 
views, says, “those portions of volcanic mountains that 
rise above the sea-level are worn down by the atmo- 
sphere and waves, and unless otherwise preserved, must 
eventually be reduced to the lower limit of effective wave- 
action, which is probably nearly coincident with the 
lower limits of reef builders.” 
I can find, however, but little further reference to it, 
and prominence has not been given to it as a principal 
cause, as has been given to, on the one hand, subsidence, 
or, on the other, the building up of mounds by organisms 
other than corals. 
Darwin specially rejects it. He says (“Coral Reefs,” 
2nd ed., p. 124): ‘‘ It will probably occur to those who 
have read Ehrenberg’s account of the reefs of the Red 
Sea, that many points in these great areas may have 
been elevated, but that, as soon as raised, the protuberant 
parts were cut off by the destroying action of the waves: 
a moment’s reflection, however, on the basin-like form of 
the atolls, will show that this is impossible ; for the up- 
heaval and subsequent abrasion of an island would 
leave a flat disc, which might become coated with coral, 
but not a deeply concave surface ; moreover, we should 
expect to see, at least in some parts, the rock of the 
foundation brought to the surface.” 
Let us now consider the general condition of the 
material piled up by a submarine volcano. 
I find that David Forbes (Geol. Mag., 1870, p. 323) is 
of opinion that erupted lava meeting water will assume 
the form of scoriz, tufa, ash, and similar loose and sub- 
divided matter, and I believe that many other geologists 
will agree. Instances of recently-formed volcanic islands 
add testimony to the correctness of this view. Graham 
Island, near Sicily, was all ash, and quickly disappeared. 
The new island in the basin of Santorin, formed in 1866, 
is all ash, as I know from personal investigation. Falcon 
Island, near the Tonga Group, which appeared in 1885, 
is all ash, and is now reduced to a small proportion of its 
original dimensions. Sabrina Island, on the flank of St. 
Miguel in the Azores, was formed in 1811, all of loose 
material, and was washed away to a depth of 15 fathoms 
in a short time. What water is now over it is not known. 
No doubt when an ash mound has assumed sufficient 
dimensions to resist the percolation of water, the lava 
will be poured out in a mass and solidify, and form a 
mountainous island of the familiar oceanic type; but it 
appears to me that all the evidence goes to show that an 
enormous proportion of the material ejected by a sub- 
marine volcano will be loose, until a great height above 
the sea is attained. 
If this be granted, here is an easy material for the sea 
to work upon. The next point is, to what depth does the 
action of the sea attain? 
To those unacquainted with the ocean it may seem 
incredible that it can be in motion sufficiently violent to, 
at depths of 50 and 60 fathoms, move material ; but I 
think that there is good evidence of it. 
An isolated rock exposed to the full strength of the sea 
from one of the great oceans will cause a heavy breaker 
on the surface, when it is submerged as much as Io 
NO. 1426, VOL. 55 | 
fathoms. Let us think fora moment what this means, 
and what the horizontal velocity of the water at the 
depth of the rock must be to cause such a disturbance at 
the surface. 
All who have studied the submarine contours of the 
land exposed to the great oceans, will know the remark- 
able fact that there is in the great majority of cases a 
sudden steeper fall at the depth of from 80 to 100 fathoms. 
This can only be explained on the supposition that the 
material eroded from the coasts can be moved and dis- 
tributed to that depth. 
The depth at which matter can be moved will, of 
course, vary with its size and tenacity. It is sufficient for 
my purpose if it is only fine mud and sand which is acted 
upon at such a depth as 80 fathoms, although submarine 
cables have been taken up which show evidence of 
having been moved and chafed at even greater depths. 
Cables have been recovered which show that breakage 
has occurred from their being moved in 260 fathoms, and, 
by the kindness of Mr. F. Lucas, 1 have in my own 
possession a steel wire forming part of the outer covering 
of the Brazilian cable, picked up from 140 fathoms, which 
is worn down on one side as with a file. The records 
of the Cable Companies can furnish numerous similar 
instances. 
While there are, as might be expected, banks in the 
oceans of every conceivable depth, there are a very large 
number with a depth over them, which is to my mind 
conformable to the depth to which wave action extends. 
I may instance the great bank on which the Seychelles 
Islands stand. This is roughly 16,000 square miles in 
area, and has a general depth over it of 30 fathoms, though 
it is not so absolutely flat as banks less gigantic. 
In the course of recent hydrographical operations it 
has been gradually borne in upon my mind that 
banks at great depth can reveal themselves upon the 
surface. Numerous instances have occurred where, on 
search being made for the cause of reported “ breakers,” 
deep banks, some lying as far below the surface as 800 
fathoms, have been found on the spot, but nothing shoaler 
could be detected. “Rips” have, however, been seen in 
the course of the search, and steered for in the expectation 
that shallow water existed, but to no purpose. In such 
cases it seems probable that it is the tide (which extends 
to the bottom of the sea) meeting the obstacle of the 
bank, which is accelerated to such an extent that it 
affects the surface, 
I have, therefore, no difficulty in believing that volcanic 
ash can be moved at depths of 30 fathoms, or more, 
when exposed to the action of waves in an otherwise 
deep sea, over which strong winds are continually 
blowing. 
The effect will be to cut down an island more or less 
rapidly, according to its constitution, to a very consider- 
able depth below the surface ; the final result being a 
perfectly flat bank. 
Mr. Darwin, as above quoted, speaks of a flat bank as 
unrepresentative of the floor of an atoll; but I think that 
this was a consequence of the comparatively small 
amount of facts at his disposal. 
I have no hesitation in saying that a flat floor is an 
invariable characteristic of a large atoll, and I cannot 
find his “deeply concave surface” in any large atoll. 
On the contrary, a flat surface is found in all of these, 
whether the rim be above or below the surface. 
It is true that towards the sides of a lagoon the depth 
gradually lessens ; the encircling rim is not so steep as it 
is on the outside, but I think this is only what would be 
expected from the less vigorous growth of coral on the 
inner side of the rim as it rises, and from the gradual 
dissemination of débris from the rim thrown over by the 
waves. 
I fail to see how subsidence of a solid peak, or the 
elevation of a submerged peak by the growth of 
