With some exceptions it is, therefore, nearly impossible to say whether an ancient object made of stone can be assigned with |12| absolute certainty to the Stone Period or no. Much will depend upon the circumstances of the discovery, and in some instances the form may be a guide. The remarks I have just made apply most particularly to the weapons, tools, and implements belonging to the period more immediately antecedent to the Bronze Age, and extending backwards in time through an unknown number of centuries. For besides the objects belonging to what was originally known by the Danish antiquaries as the Stone Period, which are usually found upon or near the surface of the soil, in encampments, on the site of ancient habitations, and in tumuli, there are others which occur in caverns beneath thick layers of stalagmite, and in ancient alluvia, in both cases usually associated with the remains of animals either locally or entirely extinct. In no case do we find any trace of metallic tools or weapons in true association with the stone implements of the old ossiferous caverns, or with those of the beds of gravel, sand, and clay deposited by the ancient rivers; and, unlike the implements found upon the surface and in graves, which in many instances are ground or polished, those from the caves, and from what are termed by geologists the Quaternary gravels, are, so far as at present known, invariably chipped only, and not ground, besides as a rule differing in form. This difference[52] in the character of the implements of the two periods, and the vast interval of time between the two, I pointed out in 1859, at the time when the discoveries of M. Boucher de Perthes, in the Valley of the Somme, first attracted the attention of English geologists and antiquaries. Since then, the necessity of subdividing what had until then been regarded as the Stone Age into two distinct stages, an earlier and a later, has been universally recognized; and Sir John Lubbock[53] has proposed to call them the Palæolithic and the Neolithic Periods respectively, terms which have met with almost general acceptance, and of which I shall avail myself in the course of this work. In speaking of the polished and other implements belonging to the time when the general surface of the country had already received its present configuration, I may, however, also occasionally make use of the synonymous term Surface Period for the Neolithic, and shall also find it convenient to treat of the Palæolithic Period under two subdivisions—those of the River-gravels and of the |13| Caves, the fauna and implements of which are not in all cases identical.