Those latter will be found worthy of ten minutes’ inspection, even as seen through the plate of glass which serves to protect them. They are selected, not as being the finest specimens in his cabinet, but as illustrating, each of them, a particular animal, or a peculiar _position_ of one in the fossil state. They are, however, very good specimens, much above the average; and if any one became missing, it could never be exactly replaced, though you should search the world over. Nature does not stamp the same “medal” _twice_. Any person who, after examining these, likes to start a theory of his own to account for their forms and colours, has my full permission to do so. But one word I would say in friendly warning, seeing that theories do abound. The Chinese, that ancient and wise people, have a theory that “asbestos” cloth may have been manufactured “of the hair of certain rats that lived in the flames of certain volcanoes.” It were monstrous to doubt it. In my beach-rambles I have often picked up and examined globes of sandstone which were partially chalcedonized, and that by evident infiltration. The lapidaries call these “sand-agates,” and reject them as unfit for the wheel; and so they are at present, but it would be instructive to meet with some of them twenty years hence, after they had undergone a more confirmed treatment at the hands of Dame Nature. The well-known specimens of “petrified wood,” common on our coasts, and occurring in some beautiful varieties of beech and acacia in the bays of South Devon, are a further example of infiltration; but the process here must have been somewhat different. The presence of metallic particles, what lapidaries term the “moss,” in many of our agates, argues an impregnating fluid, thoroughly charged with mineral matter. In most cases this fluid was a ferruginous stream, such as may often be seen issuing from some hidden reservoir, and trickling down the face of a cliff of gault or greensand. This tinges everything that lies in its path with an indelible red stain; it then oozes on through the shingle, and reaches the verge of the sand. The nearest pool becomes saturated between tides, and the suspended crystals of salt, which are of a penetrating character (as the state of your beaching-boots will soon inform you), enter into chemical combination with the metallic rust, and help to conduct its particles into the heart of many a limestone pebble. Dark inland pebbles, on the other hand, will discharge much of the oxide which they have imbibed, and may be observed to brighten and improve their complexions after a few months’ sponging and tossing in the purer sea-water. Some of our handsomer pebbles, when cut in two, reveal blotches of metal, which are glossy after polishing on the wheel. This metal is occasionally native iron. The darker varieties of “moss” contain a good deal of manganese, and some silver.