S. P. Woodward—Nature and Origin of Banded Flints. 147 
beds of gravel from which they are now cbtained. The stone 
is pellucid and stained with at least three sets of ferruginous 
lines and zones. Of these one set (7) evidently marks the succes- 
sive stages of endogenous formation, as proved by the small 
drusy cavity remaining in the centre (c). They have been 
rendered visible by the infiltration of iron-oxide in waves, not 
coinciding with the layers of structure, but crossing them in 
various directions (7). 
That some important change in the physical condition of the 
alternate layers has been produced by these infiltrations cannot 
be doubted, for in the case of our own banded flints the lamin 
are acted upon unequally by the weather. By those, however, 
who are familiar with the molecular changes produceable in 
mineral bodies this peculiarity is considered of less account. 
Sometimes the pebbles of banded flint are small, and evidently 
formed by the breaking up and bouldering of larger flints, as 
in the example repr esented by fig..2; Pl: VIL. ; and it is a rare 
thing to find a flint of any considerable size banded throughout 
with concentric lamine of colour, like the agates and Egyptian 
pebbles. The bands are for the most part local, and their form 
and direction bear no relation to the outline of the entire flint. 
The newly broken black flints from the Chalk-pits near 
Gravesend are remarkable for the frequency of discoloured 
spots and markings before referred to, especially where a Sponge 
or other imbedded substance has afforded easier access to the 
bleaching agency. These light clouds are often zoned with 
dark lines which become fewer and further apart at each end 
of the discoloured tract, and when there is an axis running 
through it the bands are curved and sometimes crescentic as in 
fic. 6, Pl. VII. The bands appear to be convex in the direction 
from which the infiltration has proceeded. The axis of the 
banded tract is often merely a dark line, as in fig. 8, PI. VII., 
where it incloses an angular fragment of the fibrous shell of 
Inoceramus; or it may be an open tube, drused with minute 
erystals, as in fig. 6; or a tube filled up by a succession of 
siliceous deposits traversed by the bands of infiltration, like the 
surrounding flint, as in figs. 4 and 3. In fig. 3 the upper part 
of the tube is filled with strings of oviform bodies (0) like those 
figured and described by Mr. Wetherell in the Mag. Nat. Hist. 
Mr. Wetherell’s cabinet contains several banded flints with 
a spongeous axis, besides the four examples now figured. One 
of them is a funnel-shaped Ventriculite (Pl. VIII., fig. 1). 
Another, more slender, had two roots which have left cavities 
passing to the outside of the flint (Pl. VIL, fig. 1). The small 
pebble, before-mentioned (Pl. VIL, fig. 2), contains a segment 
L 2 
