‘244 
THE VOYAGE OF H.M.S. CHALLENGER. 
The soluble portiou may be considered as formed of hydrated silica, argillaceous matters, 
ferruginous materials, and especially of carbonate of calcium derived from the debris of 
organisms, as in the case of the other deposits previously described. The result most clearly 
brought out from an examination of these analyses is that obtained from a consideration of 
the insoluble materials. Taking into account the percentages of the various bases, and 
remembering the silicates present in these Volcanic Muds, it is very easily seen that the in- 
soluble silica must be combined with the alumina, ferric oxide, lime, magnesia, and alkalies, 
but as the alkalies have not been estimated, it is impossible to carry these deductions further. 
However, the analyses support the conclusions arrived at from the microscopic examination 
of these sediments, which shows the existence of a large quantity of silicates of recent 
volcanic origin, and that quartz, if present, plays but a very subordinate part. 
The Volcanic JMuds and Sands are found around all the oceanic volcanic islands and 
off those coasts where volcanic rocks occur ; they are estimated to cover an area of about 
750,000 square miles, in which is included the area of the islands themselves. 
Coral Muds and Sands. 
Just as around volcanic islands the deposits are principally made up of the debris 
from volcanic rocks, so off coral islands and coral reefs the deposits are chiefly made up 
of the fragments of organisms living in the shallow waters and on the reefs, such as 
calcareous Algte, Corals, Molluscs, Polyzoa, Annelids, Echinoderms, and Foraminifera. 
These fragments form a coarse sand or gravel in the shallower waters, but beyond the 
limits of wave action there is a fine mud consisting principally of triturated particles of 
calcareous matter. With greater depth and increasing distance from the land, Pteropod 
and Ileteropod shells, as well as pelagic Foraminifera, make up more and more of the 
deposit, till the Coral Muds and Sands pass finally into a Pteropod Ooze or Globigerina 
Ooze, in which reef fragments can with difficulty be recognised. The pelagic organisms 
are, then, with difficulty detected in the deposits close to the reefs, and reef fragments 
are rare in the deeper deposits at a considerable distance from the shallow water around 
coral reefs or islands. This transition in the character of the deposits from the reef-edge 
to the deeper water of the open sea, is illustrated in the figures on Plates XIII. and XIV., 
where the deposits at various depths around the island of Bermuda and off the Fiji and 
Friendly Islands are figured. 
Coral Muds . — There are 16 Coral Muds described in the Tables of Chapter II., 
ranging in depth from 140 to 1820 fathoms, the average being 740 fathoms. 
9 are from depths under 500 fathoms. 
1 „ „ of 500 to 1000 
3 ,, ,, ,, 1000 ,, 1500 ,, 
3 „ ,, over 1500 „ 
