388 JUKES-BROWNE : MICROSCOPIC STRUCTURE OF ZONES OF CHALK. 
the chambers of Foraminifera and the axial canals of Sponge Spicules 
there seems no reason to doubt that the loose grains have been 
formed in such situations. Of detrital minerals quartz is the com- 
monest, but minute grains of other minerals also occur in some paits 
of the Chalk and in some districts : among those observed are 
Zircon, Tourmaline, Hornblende, Felspar, Rutile, and Apatite. 
Next let us consider the physical conditions which these consti- 
tuents may serve to indicate. Little can be learnt from the shells or 
shell-fragments because most of them belong to extinct species, and 
we know nothing of their habits. The Foraminifera, however, do fur- 
nish some evidence, for many of these seem to be identical with 
species now living, and the bottom-living arenaceous forms may per- 
haps be relied on as giviug some indication of the depth of water. 
The glauconite grains are more definite indicators of certain 
physical conditions. In modem deposits glauconite is formed inside 
Foraminifera shells at various depths down to 1,000 fathoms, but is 
most abundant where the deep water begins, that is from 100 to 400 
fathoms. It does not occur where large rivers enter the sea, but 
only along tracts off bold coasts where the water is comparatively 
clear. From these and other facts mentioned in the Challenger 
Reports, it appears that glauconite is formed in areas where deposition 
goes on slowly, where the amount of land-derived sediment is small, 
and where, consequently, it lies exposed for a long time to the decom- 
posing action of sea-water. An extensive deposit of glauconite sand 
or marl will, therefore, indicate clear water of from 100 to 500 
fathoms, and within 200 miles of an extensive coast-line. Moreover, 
if the accompanying quartz-grains are small it is likely to have been 
formed nearer to the 500 fathom than to the 100 fathom-line. 
The detrital minerals, quartz, felspar, zircon, &c , have generally 
been derived from the erosion of the land, and their presence in any 
quantity may usually be taken as proof of the deposit having been 
formed within 200 miles of land. Such particles, especially of the 
heavier minerals, are generally absent from the deeper oceanic 
deposits. In some regions, however, small particles of quartz do 
occur in oceanic oozes at great distances from land, and these can 
always be traced to wind-blown sands, so that the presence of minute 
