of Edinburgh , Session 1876 - 77 . 253 
A consideration of these observations, and the specimens which 
are laid on the table, will, I think, justify the conclusion that vol- 
canic materials, either in the form of pumice-stones, ashes, or other 
fragments, are universally distributed in ocean deposits. 
They have been found abundantly or otherwise in our dredgings, 
according as these have been near or far from volcanoes, or as there 
has been much or little river and coast detritus, or few or many re- 
mains of surface organisms in the deposits. 
Some of the Products of the Decomposition of Volcanic Debris. 
Clay . — Pure clay, as is well known, is a product of the decom- 
position of feldspar, and the clay which we find in ocean deposits 
appears to have had a similar origin. 
In the deposits far from land the greater part of the clay origi- 
nates, I believe, from the decomposition of the feldspar of frag- 
mental volcanic material, which we have seen to be so universally 
distributed. 
Pumice-stone is largely made up of feldspar, and from its areolar 
structure is peculiarly liable to decomposition. Being permeated 
by sea water holding carbonic acid in solution, a part of the silica 
and the alkalies are carried away, water is taken up, and a hydrated 
silicate of alumina or clay results. 
Like most clays our ocean clays contain many impurities, these 
last being as varied as the sources whence the materials of the 
deposits are derived. 
Let us briefly enumerate the sources of these materials. 
We have first the matters derived from the wear of coasts, and 
those brought to the sea by rivers, either in a state of suspension or 
solution. The material in suspension appears to be almost entirely 
deposited within two hundred miles of the land. 
Where great rivers enter the sea, and where we have strong cur- 
rents, as in the North Atlantic, some of the fine detritus may be 
carried to a greater distance, but its amount can never be very 
large. In oceans affected with floating ice we have land debris 
carried to greater distances than above stated; for instance we can 
detect such materials in the deposits of the North Atlantic as far 
south as the 40th parallel N., and in the South Pacific as far north 
as the 40th parallel S. 
