146 S. P. Woodward—Nature and Origin of Banded Flints. 
latterly entertained such a belief. It is, however, well-known 
that Dr. Bowerbank considers them fossilized sponges,—an 
opinion which few will share with him. When slit and 
polished, the banded portions shew no microscopic peculiarity 
to distinguish them from the adjacent unbanded flint, and no 
organic structure save where they invade the texture of a 
Sponge, or envelope Foraminifera, so-called Xanthidia, and 
other minute organisms with their hazy shroud. I believe 
that I can now offer conclusive evidence that the coloured 
bands in flints are produced by infiltration, as taught by the late 
Professor Henslow; a view in which Mr, Wetherell coincides 
on the proof afforded by his own specimens. 
All kinds of siliceous pebbles derived from gravel-beds are 
more or less affected by agencies to which those beds have been 
subjected, and especially they are liable to be stained by the 
penetration of water charged with iron or manganese, which 
has produced coloured zones and dendritic infiltrations in the 
most compact jaspers and agates as well as in the more per- 
meable flints derived from the waste of the Chalk-formation. 
The well-known pebbles of Egyptian jasper, when cut and, 
polished, have a dark border of ochre and umber; the centre 
creamy, with one or. many sets of parallel brown lines, en- 
croaching upon and obliterating their precursors like successive 
waves. In the ‘Crystallography and Mineralogy’ of MM. 
Mitchell and Tennant (p. 509) there is a representation of one 
of the Egyptian jaspers in the British Museum, which has 
been considered to resemble the portraits of Chaucer! We here 
represent one of the mocha-stones in the same collection. These 
were probably formed, like other agates, in the vesicular cavities 
DO ae g U g9 9 
Mocha-stone, in the British Museum ; polished section, nat. size. 
of trap-rocks; but, having been set free by the breaking up 
and decomposition of their ‘matrix, they contributed to form the 
