Each of these three Ages has various subdivisions. Those of the Age of Stone are particularly important. They are two, based upon the manner in which the stone was brought to an edge. All the specimens in geologically the oldest deposits have been brought to an edge by a process of chipping off small pieces, so as to produce a sharp line or crest on a part or the whole of the border of the stone. This artificial process leaves such peculiar traces that a practiced eye cannot confound it with any accidental chipping which natural means effect.
The later deposits of the Age of Stone show that the early workmen had acquired another manner of dressing their material; they rubbed one stone against another, thus grinding it down to a sharp polished edge.
These two methods give the names to the two periods of the Age of Stone, the Period of Chipped Stone and the Period of Polished Stone. Do not suppose, however, that the workmen in polished stone forgot the art of chipping stone. On the contrary, they continued it side by side with their new learning, and you will find on the sites of their workshops plenty of stone implements in form and technical production like the chipped implements of the older period.
We know that the polished or ground-stone implements came into use later than the earliest chipped implements, for in the oldest beds the latter are found exclusively. Hence the time when they were used exclusively is called the older stone implement period or the Palæolithic period; while, the time when both chipped and polished stones were used, metals were yet unknown, is named the newer stone implement period, or the Neolithic period. A true “Palæolith” is a typical chipped stone implement, the position of which when found leads us to believe that it was manufactured in the older of these periods.
We are not entirely dependent on its position to decide its antiquity. The kind of stone it is, the amount of weather-wearing or _patine_ it shows, certain characteristics of shape and size, the indication that the chipping was done in a peculiar manner, all these aid the skilled observer in pronouncing definitely as to whether it is a true Palæolith.
Nor is position always a guarantee of antiquity. A genuine Palæolith may have been washed into newer strata, or be exposed by natural agencies on the surface of the ground, and in such cases it may not be possible to distinguish it from the products of Neolithic industry. A recent product of art may have sunk or been buried in an ancient stratum, and thus become what is termed an “intrusive deposit.”