Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/341

 characters, being found together in particular spots. Thus those in the County of Essex, ten miles northward of London, contain a much greater proportion of argil and iron, than those met with in many other places; hence their colours are darker, and the delineations which their sections display are very strong and decided, sometimes closely agreeing with those seen in the Egyptian pebbles. Passing on into Hertfordshire, pebbles of a very different character are found: their crust is nearly black, and their section displays delicate tints of blue, red, and yellow, disposed on a dead-white ground in very beautiful forms. In another part of the same county, occurs the pebble of the pudding—stone, which also presents peculiar characters of colour, &c.

3d. Large tuberous, or rather ramose, irregularly formed flints, somewhat resembling in figure the flints which are found in chalk, materially differing however from them, not only in the colour of their external coat which is of various shades of brown; but also in that of their substance, which is seldom black, but exhibits shades of yellow or brown, in which red likewise is sometimes perceptible. The traces of organic structure, particularly of the alcyonium occasionally seen in these stones, determine them also to have been formed at the bottom of the sea.

4th. Pebbles, owing their form to an investment and impregnation, with silex, of various marine animals of unknown genera, but bearing a close affinity to the alcyonia. These stones display, in general, not only the external form, but the internal structure also of these animals. The congregation of many pebbles of this genus, and