Either a fluid menstruum, usually siliceous, must have enveloped and saturated the animal form, or there was actual injection of chalcedony and limestone, in a soft state, into the tubes and cells of the skeleton first, and afterwards into the pores and crevices of the new stone. Frequently both processes have obtained. The only alternative, viz. that which suggests that composite pebbles, revealing in their structure distinct traces of animal or vegetable organization, _may have been_ formed thus at first, like the coloured prints in a book, I consider to be untenable. One might as readily credit a spontaneous growth of almond-cakes or oyster-patties by some sudden spasmodic effort of our present sea and land. Impregnation has, no doubt, been always going on. A French _savant_, M. Reaumur, above a century ago, wrote as follows:—“By a coarse operation emery is reduced to powder and suspended in water for several days; but nature may go much further than this, for the particles which water detaches from hard stones, by simple attrition, are of an almost inconceivable degree of fineness. Water thus impregnated contributes to the formation of pebbles by petrifying the stone, as it were, a second time. Stones already formed, but having as yet a spongy texture, acquire a flinty hardness by impregnation with this crystalline fluid.”[3] From such a source, as he supposes, has arisen the close texture of Egyptian pebbles, coloured jaspers, and even agates. If he means to include the _homogeneous_ agates, which are alike free from sparry crystals, metallic scum, and all traces of organized matter, I do not agree with him. But in the case of _mixed_ chalcedonic pebbles, among which may be classed our pretty Isle of Wight specimens, no doubt such impregnation took place, and was followed by a further process of infiltration. For when the flint-nodules, _impregnated_ as above, were still soft—soft enough we know they were to take delicate impressions of the spines of the echinus—chalcedony, in a semi-fluid or viscous state, would pass through the pores of the flint, because the former is the finer substance of the two. And after such infiltration, the entire lump would harden, resisting, for the most part, further change. And this would be the pebble as we now find it. In the “Geological Museum,” now open in Jermyn Street, there is a case (on the first floor) where the nature of infiltration is well shown in some jasper-agates. It will readily be seen here that each internal layer has been formed in succession _from without_, the centre of the pebble filling up last. In another case (labelled “Silica”), on the opposite side of this room, are some “choanites” and “sponges” presented by the author.