On the Structure of some of the Siliceous Nodules of the Chalk

Abstract

Abstarct T he author described several specimens of the peculiar banded flints* found in the chalk and in gravel, and of which he had made a large collection during several years. They usually exhibit a central longitudinal axis or narrow stem, crossed on its middle third by numerous short parallel stripes of alternately light and dark flint, and frequently terminated at each extremity by an irregular mass of flint, often clouded or grey. The axis occurs sometimes isolated, sometimes covered with a thin coating of grey flint only, and sometimes associated with only a few cross stripes of the banded structure. In some instances the banded flint has for its axis a sponge, or fragments of sponge. The author had not found in the banded flint any spongy tissue peculiar to it; in some instances, however, a silicified sponge appears to have been traversed by alternate lines of the light and dark colour analogous to those of the banded flints. In some instances a concentric arrangement of light and dark layers of flint occurs around the two ends of an axis, or around isolated nuclei. Mr. Wetherell regarded this banded appearance in the flint as not being due to an organic structure, but to have originated in a peculiar arrangement of the siliceous matter around organic bodies, frequently long and stem-like, such as those of the Graphularia , which supplied so many axial nuclei to the concretions in the London Clay†.