This one can see by the traces of conchoidal fracture which they all show.
The art is not wholly understood, for it perished soon after the Conquest, when iron came in; but, as far as the theory is concerned, I think I can give a tolerably satisfactory account of the process of manufacture.

In the first place, the workman who makes gun-flints could probably make some of the simpler obsidian implements, which were no doubt chipped off in the same way.

The section of a gun-flint, with its one side flat for sharpness and the other side ribbed for strength, is one of the characteristics of obsidian knives.

That the flint knives of Scandinavia were made by chipping off strips from a mass is proved by the many-sided prisms occasionally found there, and particularly by that one which was discovered just where it had been worked, with the knives chipped off it lying close by, and fitting accurately into their places upon it.
Now to make the case complete, we ought to find such prisms in Mexico; and, accordingly, some months ago, when I examined the splendid Mexican collection of Mr.Uhde at Heidelberg, I found one or two.

No one seemed to have suspected their real nature, and they had been classed as maces, or the handles of some kind of weapon.
[Illustration: FLUTED PRISM OF OBSIDIAN: THE CORE FROM WHICH FLAKES HAVE BEEN STRUCK OFF.] I should say from memory that they were seven or eight inches long, and as large as one could conveniently grasp; and one or both of them, as if to remove all doubt as to what they were, had the stripping off of ribbons not carried quite round them, but leaving an intermediate strip rough.