234 Proceedings of Royal Society of Edinburgh. [sess. 
about 6 per cent., and in the Southern and Antarctic Oceans 
about 16 per cent., of the deposits. In some special regions, how- 
ever, they play a much more important part in the formation of deep- 
sea deposits. There would appear to be a wide band or zone of 
Diatom Ooze surrounding the South Pole, between the latitude of 
40° S. and the Antarctic Circle, covering about 10,880,000 square 
miles of the sea-bottom, in which the percentage of siliceous organisms 
is, on the average, about 50 per cent, of the whole deposit. 
Again, in the central parts of the Pacific and Indian Oceans there 
are large areas of Radiolarian Ooze in the greatest depths, covering 
in all about 2,290,000 square miles of the earth’s surface, in which 
the remains of siliceous organisms are estimated to make up nearly 
60 per cent, of the whole deposit. In some Globigerina and Pteropod 
Oozes, as well as in some Red Clays and Blue Muds, there are in 
certain regions very few, if any, traces of these pelagic Diatoms and 
Radiolaria. It is somewhat difficult to account for their absence, 
for they are captured in the surface waters of these areas, although 
not in such great abundance as in the surface waters over regions 
where they make up a large part of the deposit at the sea-bottom. 
In some instances they appear to have been removed in solution, 
as will be pointed out later on; in others their presence may be 
masked by the relatively much more rapid accumulation of cal- 
careous remains, of triturated pumice, or of land debris. 
In our previous paper, when discussing the secretion of carbonate 
of lime by marine organisms, we found it impossible to accept the view 
that the lime was absorbed and secreted directly as carbonate, owing 
to the small amount of carbonate of lime in solution in sea-water — 
one part in 8000 — and the enormous quantity of water that would 
consequently require to pass into the life circulation to permit its 
secretion were this the only source. We were able to show, we 
think successfully, that organisms may obtain their carbonate of lime 
from any of the lime salts in sea-water, by means of the changes 
produced by their effete or waste products on the constitution of the 
lime salts in solution, the whole of the lime present being in this 
way available for the formation of carbonate of lime shells. A 
similar difficulty with reference to the source of the silica is pre- 
sented in the case of the silica-secreting organisms, for to obtain the 
