386 
THE VOYAGE OF H.M.S. CHALLENGER. 
mineral and the peculiarities of its occurrence, it is impossible to be sure that the analyst 
has always been dealing with a pure substance ; indeed, microscopic examination of 
Siimples that have been submitted to analysis shows that there is often an admixture of 
foreign mineral particles, and that the individual grains of glauconite in the sample do not 
always present uniform external characters ; this in all probability accounts largely for 
the variable composition. The following analyses of glauconite formed in modern seas 
present in their principal features a great analogy with the composition of the same 
mineral in geological formations. Amongst the numerous analyses of glauconite that of 
a specimen of the chalk of New Jersey by Sterry Hunt^ gives the closest approximation 
to the figures obtained in analysing what we consider typical modern glauconite (Station 
164b, Nos. 86, 87). But a glance at the analyses shows how much this mineral may vary 
in composition, although the physical characters seem to be the same. These divergences 
are more marked when we compare the figures of the analyses of paler grains associated 
with the darker ones at the same station (No. 84 compared with Nos. 86 and 87). All 
that can be said is that the glauconite now forming on the bottom of the sea is, like the 
glauconite of geological formations, a hydrous silicate of potash and of ferric oxide, contain- 
ing always variable quantities of alumina, ferrous oxide, magnesia, and often lime. If we 
compare the figures of the two analyses with the mean composition of glauconite given by 
Haushofer,^ we see that all the percentages, except those of silica and perhaps water, difier 
greatly from the figures given in our analyses. The analysis No. 88 resembles that of de- 
composed glauconite, and the composition of this specimen may be compared with that of 
the altered glauconite of Kressenberg, given by Haushofer® ; the high percentages of per- 
oxide of iron and water point to a decomposition of this mineral which has been transformed 
into limonite, as is often the case in glauconite from the geological strata, with loss of silicic 
acid and of pota,sh, but this interpretation can hardly be given for this specimen, which 
consisted of casts from a Coral Sand off the Great Barrier Reef of Australia. There can 
be no doubt that glauconite is a mixture, and this fact not only renders it difficult to fix 
its constitution, but also renders difficult any interpretation of the mode of formation. 
We give here the analyses of some specimens of glauconite collected during the 
expedition. The substance used for Analysis No. 84 contained 65 per cent, of white, pale 
grey, and some yellow casts, 20 per cent, of pale green casts, 11 per cent, of dark green 
casts, along with 4 per cent, of mineral jiarticles and siliceous organisms. The substance 
used for Analysis No, 8.5 contained 15 per cent, of white, pale grey, and yellow casts, 
35 per cent, of pale green casts, 45 per cent, of dark green particles, together with 5 per 
cent, of mineral particles and siliceous organisms. The substance used for Analysis 
No. 86 contained 10 per cent, of white, pale grey, and yellow casts, 25 per cent, of pale 
* Sterry Hunt, Minernl Pliysiology and Physiography, p. 198, Boston, 1886. 
* Hausliofer, Journal f. jrrnkt. Chemie, Bd. xcvii. pp. 363-364, and Bd. xeix. pp. 237-8, 1666. 
iiausliofer, loc. cit., Bd. xcvii. p. 368. 
