hard material. The flint is white and porcellanous on the surface, and has become so light and soft in structure, that it can readily be cut with a knife. It was found in the south-west chamber of the cavern, beneath stalagmite not quite a foot thick, but touching the ceiling of the chamber, or nearly so, in company with teeth of hyæna, bear, and fox, and a small quartz crystal.
With regard to this alteration in the colour and structure of the flint, it may be well here to make a few remarks. At first sight, it seems difficult to believe that in a material so hard, and under ordinary circumstances so extremely durable, as flint, so complete a change in colour and texture should have taken place, during any lapse of time, however great. We find, however, that under certain circumstances, even Neolithic implements, which still retain their original black or dark colour in the interior, have on their exterior become completely whitened, and in some cases softened so much that they can be scratched with a knife. The cause, as was first pointed out to me by the late M. Meillet,[1] of Poitiers, appears to be inherent in the nature of most flints, the silica in which is of two kinds; the one crystallized silica or quartz, with a specific gravity of 2⋅6, and insoluble in water, the other colloid or glassy silica, known as opal, with a specific gravity of 2⋅2, which is much more transparent, horny, and soluble; though in their other properties both are chemically the same. It appears, then, that in these whitened flints, the soluble portion has been removed by the passage of infiltrating water through the body of the flint, while the insoluble portion has been left in a finely- divided state, consisting of particles susceptible of disaggregation by moderate force, and is consequently white. This alteration in structure is not confined to artificially-wrought flints, but may take place even in flint pebbles, under certain circumstances, in pervious soils; for I have found Lower Tertiary pebbles in the Woolwich and Reading beds, and also in the resulting conglomerates, which have become sufficiently disintegrated to be cut with a steel knife. When it is considered that these pebbles were originally the hardest part of chalk flints, or at all events those parts which were best able to withstand the rolling and wearing action of the Tertiary sea, the amount of alteration they have since undergone, by the slow dissolution of a portion of their sub-
- ↑ "Recherches Chimiques sur la Patine des Silex tallés." Montauban, 1866. See also Judd, in Proc. Geol. Assoc., vol. x. p. 218, and Lobley, op. cit., p. 226; as also Comptes Rendus de l'Ac. des Sc., 1875, p. 979.
