668 
Proceedings of the Royal Society 
Siliceous organisms [3 ’00] : Sponge spicules, Diatoms and Radio- 
larians, and one or two pale glauconitic casts. 
Fine washings [19 '25] ; argillaceous matter with minute fragments 
of minerals, diatoms, and other siliceous organisms. 
Remarks . — When the mud is passed between the fingers many 
gritty particles are found to be present. These can be observed 
generally to be fragments of quartz and rocks. 
Chemical analysis gave the following : — 
The carbonate of lime appears to make up about one-fourth of the 
deposit in bulk, and consists almost wholly of pelagic Foraminifera, 
Coccoliths, and Coccospheres, the same as found at the stations in 
the warm area. 
Mixed up with these are a few of the bottom living Foraminifera 
and fragments of Echinoderms. The insoluble residue is essentially 
selves, the difficulty is still greater, on account of their isolated condition and 
the physico-chemical actions which they have undergone. However, we 
are often aided by their association with fragments of rock, whose classification 
is possible ; by their being sometimes seen, on microscopic examination, to be 
bordered by remains of the rock of which tliejr were formerly an integral part ; 
and by their geographical position (which point is also applicable to the deter- 
mination of the age of fragments of rocks). Whatever we may think of the 
causes of the characteristic differences of these two lithologic groups, this 
distinction offers, none the less, a very practical advantage, and if it is 
applied with care, it throws some light on the interpretation of the facts with 
which we have to deal. 
We cannot dwell, in this note, upon the diagnostic characters which we have 
employed ; we will return to that in the introduction to our Report on the 
deposits collected during the “Challenger” expedition. As we have had, 
especially, to study very minute fragments of rocks and of isolated minerals, 
questions relative to the structure of rocks, properly so called, have been 
considered only in a subsidiary manner. We give some of the characters 
which have guided us in the classification into the two groups. We repeat 
that these peculiarities have only a relative value, and we do not regard them 
as absolutely conclusive in every case. 
We may add that it is especially on the microscopic examination that we 
have relied for our determinations. 
Column I. presents the characters of the principal minerals of ancient rocks 
Insoluble in HC1, 
84-55 
1-22 
14-34 
A1.,0 3 - Fe 9 0 3 , 
CaC0 3 - MgCOg , 
100-11 
