Secondly, such parts of our coast as are hemmed in by cretaceous or sandstone cliffs abound in fossil pebbles. The organisms which these inclose are, almost universally, those of zoophytes. Dendritic or vegetable markings are rare, occurring principally in the white chalcedony. A good pebble of this kind used to be called a mocha-stone; the difficulty always was how to be quite sure where it came from. Those from the Indies would generally be handsomer; while, on the other hand, an equally fine specimen from our own shores would be of treble value.
But the petrifactions described in Chapter II. may be readily obtained throughout the entire range of coast from Hastings to Selsea Bill, and further.
In gathering these pebbles, if you intend them for the cabinet, and not merely to amuse a passing hour, regard should be had to three points; the size, the pattern, and the colour. The last-mentioned, which is, nine times out of ten, the cause of average specimens being seen at all, will itself be determined by the material of which the pebble consists. Chalcedony has a bluish cast in the solid stone, especially when it is wet with brine; but where “moss” is present, this will cause a golden or red tint. Jasper, when semi-transparent, is greenish, otherwise, a blood-red.
As to size, choanites and the globular “sponges” will vary from that of a small pippin to that of a full-sized orange. Very rarely they exceed this. If the pebble be of the right sort, the larger the better: but, as a general rule, the finer ones run small.
The pattern is the most important feature; as you will soon recognize in making a collection. And this, moreover, if no injury has yet been done to the stone, decides the “contour” of the rolling beauty.
I shall not, however, say much about these patterns; not wishing to be charged with exaggeration, as I probably should be, by those who have never seen good ones, and who do not conceive of the endless varieties into which four or five colours can thus be wrought in the stony loom of Nature.
Amateurs must learn for themselves what these really are, and how to judge of them scientifically, when they pick up a fossil weighing, perhaps, a pound avoirdupois, and looking like a champion-potato. One or two hints may be ventured. If the creature was an “alcyonite,” its facsimile in stone should have the external rings or mouths clearly defined, and, if possible, equi-distant. Such a one, when cut, will exhibit its tubes evenly disposed—not unlike a section of fir-wood under the microscope. If it was an “actinia,” then the body of the zoophyte should be central in the pebble, and the tentacles will be not much melted away in a good slice.
“Sponges” must be chosen principally for their colour; in other words, for the texture of the stone. Those which exhibit the reticulations, white or straw-colour, upon a very dark ground, are the most effective generally: but the most perfect one I ever saw had a blood-red pattern upon white. For a single slice from this stone, half a guinea was offered by me, and refused.
“Choanites” are easily discerned.
The “ventriculite” must have been a creature lower down in that scale than the choanite. This fossil is repeatedly met with on our coast, but I do not admire it enough to have retained a single specimen. When alive, it would appear to have resembled, in stature and configuration, an ordinary toadstool.