The ancient flints present a glassy surface which singularly contrasts with the dull appearance of the fresh cleavages. They are also for the most part covered with a whitish coating or _patina_, which is nothing but a thin layer of carbonate of lime darkened in colour by the action of time. Lastly, many of these flints are ornamented with branching crystallisations, called _dendrites_, which form on their surface very delicate designs of a dark brown; these are owing to the combined action of the oxides of iron and manganese (fig. 8). We must add that these flint implements often assume the colour of the soil in which they have been buried for so many centuries; and as Mr. Prestwich, a learned English geologist, well remarks, this agreement in colour indicates that they have remained a very considerable time in the stratum which contains them. Among the stone implements of primitive ages, some are found in a state of perfect preservation, which clearly bears witness to their almost unused state; others, on the contrary, are worn, rounded, and blunted, sometimes because they have done good service in bygone days, and sometimes because they have been many times rolled over and rubbed by diluvial waters, the action of which has produced this result. Some, too, are met with which are broken, and nothing of them remains but mere vestiges. In a general way, they are completely covered with a very thick coating which it is necessary to break off before they can be laid open to view. They are especially found under the soil in grottos and caves, on which we shall remark further in some detail, and they are almost always mixed up with the bones of extinct mammalian species. Certain districts which are entirely devoid of caves contain, however, considerable deposits of these stone implements. We may mention in this category the alluvial quarternary beds of the valley of the Somme, known under the name of drift beds, which were worked by Boucher de Perthes with an equal amount of perseverance and success. [Illustration: Fig. 9.--Section of a Gravel Quarry at Saint-Acheul, which contained the wrought Flints found by Boucher de Perthes.] This alluvium was composed of a gravelly deposit, which geologists refer to the great inundations which, during the epoch of the great bear and the mammoth, gave to Europe, by hollowing out its valleys, its present vertical outline. The excavations in the sand and gravel near Amiens and Abbeville, which were directed with so much intelligence by Boucher de Perthes, have been the means of exhuming thousands of worked flints, affording unquestionable testimony of the existence of man during the quaternary epoch. All these worked flints may be classed under some of the principal types, from which their intended use may be approximately conjectured.