Manual skill had, however, attained such a pitch of perfection among the workmen of this period, in consequence of their being habituated to one exclusive kind of labour, that the nature of the stone became a matter of indifference to them. The hammer, with the proper use of which our workmen are almost unacquainted, was a marvellous instrument in the hands of our ancestors; with it they executed prodigies of workmanship, which seem as if they ought to have been reserved for the file and grindstone of the lapidary of the present day. We shall not, perhaps, surprise our readers if we add that as certain volcanic lavas, especially obsidian, fracture with the same regularity and the same facility as the flint, obsidian was employed by the natives of America as a material for making sharp instruments. The ancient quarries whence the Indians procured this rock for the manufacture of instruments and tools, were situate at the _Cerro de Navajas_--that is, the _Mountain of Knives_--in Mexico. M. H. de Saussure, the descendant of the great geologist, was fortunate enough to meet with, at this spot, pieces of mineral which had merely been begun upon, and allowed a series of double-edged blades to be subsequently cut off them; these were always to be obtained by a simple blow skilfully applied. According to M. H. de Saussure, the first fashioning of these implements was confined to producing a large six-sided prism, the vertical corners of which were regularly and successively hewn off, until the piece left, or _nucleus_, became too small for the operation to be further continued. Hernandez, the Spanish historian, states that he has seen 100 blades an hour manufactured in this way. Added to this, the ancient aborigines of Peru, and the Guanches of Teneriffe, likewise carved out of obsidian both darts and poniards. And, lastly, we must not omit to mention that M. Place, one of the explorers of Nineveh, found on the site of this ancient city, knives of obsidian, supposed to be used for the purpose of circumcision. Having considered the flint instruments peculiar to the polished-stone epoch, we must now turn our attention to those made of stag's horn. The valley of the Somme, which has furnished such convincing proof of the co-existence of man with the great mammals of extinct species, is a no less precious repository for instruments of stag's horn belonging to the polished-stone epoch. The vast peat-bogs of this region are the localities where these relics have been chiefly found. Boucher de Perthes collected a considerable number of them in the neighbourhood of Abbeville.