though it will be well again to call attention to some of the distinctive marks by which artificially formed flakes may be distinguished from mere splinters of natural origin. The formation of these latter is usually due either to the flint, while still embedded in the chalk, having received some violent shock from disturbance of the stratum; or to unequal expansion, which sometimes causes flints to split up into rudely prismatic forms, much like those assumed by starch in drying, and sometimes causes cracks on the surface, which enable water and frost to complete the work of splitting them. Occasionally, nearly flat planes of fissure are caused by the expansion of some small included particle of a different mineralogical character from the surrounding flint. In such cases a series of concentric and more or less circular rings may usually be traced on the surface surrounding the central particle, which apparently mark the intervals of repose, when its expansion had ceased for a time to exert sufficient force to continue the fissure. This kind of fracture is most prevalent in flints upon or near the surface of the ground, such as those in drift-deposits.
In hardly any instances of natural fracture does the surface of the splinter show any trace of its having been produced by a blow, though the violent impact of one stone upon another, by means of a fall from a cliff, or of other natural causes, might produce a splinter of the same form as if it had been struck off by a hammer. There would, however, be the mark of the blow on one face only of such a splinter, whereas in a perfectly artificial flake the traces of the blow by which each facet was produced would be discernible. On the sea-shore, natural splinters of flint, resulting from the blow of one wave-borne pebble on another, may occasionally be found, some of them having a kind of secondary working at the edges, the result of attrition among the pebbles on the shore.
If a blow from a spherical-ended hammer be delivered at right angles on a large flat surface of flint, the part struck is only a minute portion of the surface, which may be represented by a circle of very small diameter. If flint were malleable, instead of being slightly elastic, a dent would be produced at the spot; but, being elastic, this small circle is driven slightly inwards into the body of the flint, and the result is that a circular fissure is produced between that part of the flint which is condensed for the moment by the blow, and that part which is left untouched. As each particle in the small circle on which the hammer impinges may be considered to rest on more than one other particle, it is
