At some points the limestone has been dolomitized; near Bonsall it has been converted into a granular silicified rock.
It has the appearance of a delicate tube which has granular contents, and is provided withani apexthatappears to be open.
It is generally surrounded by a granular or radiating cytoplasmic substance.
If the tin is pure it splits into a mass of granular strings.
Two layers are specially obvious in its walls - the inner layer bordering the lumen being composed of smaller ciliated cells, the outer thicker one containing numerous granular cells and having a more glandular character.