These peat-bogs are, as is well known, former marshes which have been gradually filled up by the growth of peat-moss (sphagnum), which, mixed with fallen leaves, wood, &c., and being slowly rotted by the surrounding water, became converted after a certain time into that kind of combustible matter which is called peat. The bogs in the valley of the Somme in some places attain to the depth of 34 feet. In the lower beds of this peat are found the weapons, the tools, and the ornaments of the polished-stone epoch. Among these ancient relics we must mention one very interesting class; it is that formed by the association of two distinct component parts, such as stone and stag's horn, or stone and bone. The hatchets of this type are particularly remarkable; they consist of a piece of polished flint half buried in a kind of sheath of stag's horn, either polished or rough as the case may be (fig. 111). [Illustration: Fig. 111.--Polished Flint Hatchet, with a Sheath of Stag's Horn fitted for a Handle.] The middle of this sheath is generally perforated with a round or oval hole intended to receive a handle of oak, birch, or some other kind of wood adapted for such a use. Fig. 112, taken from the illustration in Boucher de Perthes' work ('Antiquités Celtiques et Antédiluviennes'), represents this hatchet fitted into a handle made of oak. [Illustration: Fig. 112.--Flint Hatchet fitted into a Stag's-horn Sheath, having an Oak Handle, from Boucher de Perthes' illustration.] It is difficult to understand how it was that a hatchet of this kind did not fall out of its sheath in consequence of any moderately violent blow; for it seems as if there was nothing to hold it in its place. This observation especially applies to hatchets, the whole length of which--even the portion covered by the sheath--was polished; for the latter would certainly slide out of their casing with ease. The fact is, that complete specimens are seldom found, and, generally speaking, the flints are separated from their sheaths. With regard to the handles, the nature of the material they were made from was unfavourable to their preservation through a long course of centuries; it is, therefore, only exceptionally that we meet with them, and even then they are always defaced. Fig. 113 is given by Boucher de Perthes, in his 'Antiquités Celtiques,' as the representation of an oaken handle found by him. A number of these sheaths have been found, which were provided at the end opposite to the stone hatchet with strong and pointed teeth. These are boar's tusks, firmly buried in the stag's horn. These instruments therefore fulfilled a double purpose; they cut or crushed with one end and pierced with the other. Sheaths are also found which are not only provided with the boar's tusks, but are hollowed out at each end so as to hold two flint hatchets at once. This is represented in fig. 114 from one of Boucher de Perthes' illustrations.