513 
of Edinhurgli, Session 1883 -84. 
a greater or less extent from the bottom. It is in the central 
regions of the Pacific that we meet with the typical examples. 
Like other marine deposits, this one passes laterally, according 
to position and depth, into the adjacent kinds of deep-sea ooze 
or mud. 
The argillaceous matters are of a more or less deep brown tint from 
the presence of the oxides of iron and manganese. In the typical 
examples no mineralogical species can be distinguished by the 
naked eye, for the grains are exceedingly fine and of nearly uniform 
dimensions, rarely exceeding 0‘05 mm. in diameter. It is plastic 
and greasy to the touch ; when dried it coagulates into lumps so 
coherent that considerable force must be employed to break them. 
It gives the brilliant streak of clay, and breaks down in water. The 
pyrognostic properties show that we are not dealing with a pure 
clay, for it fuses easily before the blow-pipe into a magnetic bead. 
Under the term red clay are comprised those deposits in which the 
characters of clay are not well pronounced, but which are mainly com- 
posed of minute particles of pumice and other volcanic material 
which, owing to their relatively recent deposition, have not undergone 
great alteration. If we calculate the analyses of red clay it will be 
seen, moreover, that the silicate of alumina present as clay 
( 2 Si 02 ,Al 203 -t- 2 H 2 O) comprises only a relatively small portion of 
the sediment, the calculation shows always an excess of free silica, 
which is attributed chiefly to the presence of siliceous organisms. 
Microscopic examination shows that a red clay consists of 
argillaceous matter, minute mineral particles, and fragments of 
siliceous organisms ; in a word, it is in all respects identical 
with the residue of the organic oozes. The mineral particles 
are for the greater part of volcanic origin, except in those cases 
where continental matters are transported by floating ice, or 
wdiere the sand of deserts has been carried to great distances by 
winds. These volcanic minerals are the same constituent minerals 
of modern eruptive rocks, enumerated in the description of volcanic 
muds and sands; in the great majority of cases they are accom- 
panied by fragments of lapilli and of pumice more or less altered. 
Vitreous volcanic matters belonging to the acid and basic series of 
rocks predominate in the regions where the red clay has its greatest 
development, and it will be seen presently that the most character- 
