889 
of Edinburgh, Session 1885 - 86 . 
of some lagoons and lagoon-channels. I should add that Mr 
Darwin also arrived at the conclusion that local conditions, such as 
the degree of clearness of the water and the extent of inclined slope, 
might partly determine the depth of the coral zone* but he did 
not attribute the same importance to such causes that I have done, 
and thus he came to seek for other explanations of the anomalous 
depths within some atolls and barrier-reefs. 
My observations on the recent calcareous formations of the 
Solomon Group f enable me to approach this subject by another 
road. These investigations have shown that coral-reefs are based 
usually on a partially consolidated volcanic mud or ooze often 
foraminiferous, generally abounding with recent shells, and now 
and then laden with pteropod-shells in considerable numbers, the 
thickness of the overlying coral-rock rarely exceeding a hundred 
feet. That the reef-corals commence to grow on such a bottom, and 
not on a layer of detritus of sand and gravel, is shown by the fact 
of my finding at Santa Anna two massive corals of the Astraeidse, 
the largest 4 feet in diameter, imbedded in the position of growth, 
at a height of 40 feet above the sea, in the base of a coral-lime- 
stone cliff where they almost rested on the subjacent partially 
consolidated ooze. It is a noteworthy circumstance that in my 
numerous soundings off the outer edge of reefs in this group, which 
extended to fifty fathoms, the armings never brought up any other 
indication of the nature of the bottom, outside the usually accepted 
coral-zone, than that of calcareous sand and gravel. In truth, my 
soundings down to depths of 50 fathoms failed to reach the ooze. 
It would therefore appear that such reefs as those of the Shortland 
Islands commenced to build in depths greater than fifty fathoms. 
If elevation had brought the ooze within these depths uncovered by 
the calcareous detritus, the armings would probably have recorded 
such an occurrence amongst some of my numerous soundings. 
An apparent objection here presents itself. If reefs begin to 
build their foundations in depths greater than those which are 
generally assigned to them, the thickness of the elevated reef- 
formation discovered by me in the Solomon Group should have been 
much greater than 150 feet, the actual limit of their thickness. It 
* Coral Beefs (1842), p. 84. 
t Vide Trans. Edin. Boy. Soc., vol. xxxii. part iii. p. 545. 
