98 
Proceedings of P,oyal Society of Edinburgh. 
there. After reaching the bottom more of the shell may he dis- 
solved, but in these positions there are circumstances which retard 
solution. Where shells reach the bottom in depths of a few 
hundred fathoms, they are soon covered up by other shells, and are 
surrounded by sea water which is completely or nearly saturated 
with carbonate of lime, from its being but slowly renewed, and 
from having been for a long time in contact with a wide extent 
of calcareous deposits at the bottom of the sea. It is probable that, 
say, at a depth of 500 fathoms, fully 80 per cent, of the carbonate 
of lime shells that fall from the surface reach the bottom and 
accumulate there. 
At depths, however, of 1500 or 2000 fathoms not more than 50 
per cent, of these shells reach the bottom in a partially dissolved 
condition ; moreover, the accumulation is here slower, from a greater 
number of shells being dissolved in their fall through the water, 
and after reaching the bottom the shell will be for a longer time 
exposed to the action of water before being covered up and pro- 
tected by the fall of other shells, as is the case in shallower depths. 
In the greatest depths of the ocean all the surface shells appear to 
be removed before they reach the bottom, except probably some of 
the heavier and more compact ones, which lie there uncovered by 
other shells till wholly removed by solution. The water in these 
red-clay areas would, besides, take up the lime more readily, as 
there it is not in contact with great carbonate of lime deposits, 
like the water overlying a Globigerina or Pteropod ooze. These 
deeper layers of water are more active in the removal of carbonate 
of lime than the surface waters, not only because of the larger 
amount of carbonic acid they contain, and to the deoxidation of 
alkaline sulphates by organic matter giving rise to hydrosulphuric 
acid, but also because of the greater pressure, and the fact that, 
the substance of the shells being less compressible than water, they 
would fall more slowly, and hence would be longer exposed to the 
action of the deeper layers of water than to the action of those 
nearer the surface. 
In this way we appear to have a perfectly rational explanation of 
the partial disappearance of carbonate of lime shells from the 
shallower depths, and their total disappearance from all the greater 
depths of the ocean. It is to be observed that all those shells in 
