18 
THE VOYAGE OF H.M.S. CHALLENGER. 
calcium as determined by analysis. The numbers within brackets in each of the other 
columns before siliceous organisms, minerals, and fine washings, give the estimated 
abundance of these in the deposit, as in the case of the relative abundance of the various 
carbonate of lime organisms under the heading carbonate of calcium ; in short, all the 
numbers in the tables within brackets ( ) represent the estimated quantity present after 
examination, in contradistinction to those numbers without brackets, which are the result 
of quantitative determination. 
1. Siliceous Organisms. — These consist generally of Diatoms, Radiolaria, and Sponge 
spicules of various kinds. The enumeration of the genera of these would much exceed 
the limits necessarily imposed by the style of the table, but full particulars are given in 
the special reports, to which the reader is referred. Under this heading are also grouped 
those Foraminifera, the tests of which are for the most part made up of the inorganic 
particles found in a deposit, like the Astrorhizidse, Lituolidse, &c. There are also placed 
under this heading the glauconitic and other casts of marine organisms which are 
occasionally found in considerable quantity in some deposits, and remain in the residue 
after the removal of the carbonate of calcium ; these cannot, of course, be classed 
among siliceous organisms in the same sense as Diatoms, Radiolarians, and Sponge 
spicules, but their mineralogical characters are indistinct, and for descriptive purposes 
this appears the best place to note their occurrence. These casts bear distinctly the 
impress of the calcareous organisms, but their chemical composition has not been in all 
wises determined, though in many instances they are probably either a silicate corre- 
sponding to glauconite, or of a phosphatic nature. When glauconite is present with 
well-defined characters, it is placed among the mineral particles. 
2. Minerals. — The fragments of minerals and rocks were examined on slides, first 
by reflected light, dry, and then by transmitted light, under the mineralogical microscope. 
Sometimes preparations were mounted in Canada balsam, or the particles were cemented 
together by gum copal, and then rubbed down till thin and transparent, by a process 
analogous to that u.sed in making thin sections of rocks.^ But this was possible only in 
certiiin eases, and generally we had to examine the mineral fragments on a slide with 
water, this mode of observation allowing the particles to remain free, and rendering easy 
chemiwil reactions under the microscope. The mean diameter and form of the mineral 
fnigoncnts arc stated in all cases, as this is a matter of considerable importance in giving 
a clue as to the agents at work during the formation of the sediment under con- 
sidcration. The order in which the species are mentioned in the tables is, generally 
speaking, that of their imjjortance or abundance in the deposits. The characters which 
have guided us in diagnosing the mineral species most commonly met with in the deep- 
sea deposits are briefly stated below ; the characters noted under the various species refer 
• Hc« F. a. Pcarwy, Mctlirxl of ConRr>liduting and Prejinring Thin Sections of Friable and Decomposed Rocks, 
Sandjs Clays, Oozes, and other Granulated Substances, Proc. Roy. Phys. ISoc. Edin., vol. viiL pp. 296-300, pi. xi., 1884. 
