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Rh formed; for, especially when they were undoubtedly human habitations, the caves seem to have been under more favourable conditions both for the reception and the preservation of a greater proportion of the smaller forms of instruments than the River-drifts; but their comparative scarcity in the collections formed from the latter is also no doubt partly due to the difficulty in finding such minute objects when imbedded in a mass of gravel, even had they remained uninjured in the course of its deposition. On the other hand, the rarity of the larger forms of implements in the cave deposits, appears to be due to these instruments having been mainly used for what may be termed "out of doors" purposes.

Again, though in some instances the River-drift and Cave-deposits belong apparently to the same period, yet in others it seems possible that we have, in the caves, relics derived from a period alike unrepresented in the old alluvia and in the superficial soil; and which may belong to an intermediate age, and thus possibly assist, especially in the case of some caves in the neighbourhood of Mentone, to bridge over the gap that would otherwise intervene between the River-drift and the Surface Period. It is not, indeed, in our English caves, that such good evidence of a sequence in the order of the deposition of their contents can be observed, as in those of the south of France, and of Belgium, in which a sort of chronological succession has been pointed out by M. Gabriel de Mortillet and others, as will subsequently be seen. It will of course be understood that this sequence in no way refers to the occupation of caverns by man in modern, or even Neolithic times. Many caves in this, as in other countries, have been the retreats or dwelling-places of man at various, and often very remote, periods: though subsequent to the time when their earlier contents had been sealed up beneath a layer of stalagmite, itself a work of centuries of slow deposition of carbonate of lime held in solution by water infiltrating from above. It is owing to the occasional admixture of the more recent remains with those of older date, either in the progress of the excavation of the caverns, or by the burrowing of animals, or in some cases possibly by pits having been sunk in the floor of the cave by some of its successive human occupants, that doubt has been thrown in former times on the value of the evidence afforded by cavern-deposits, as to the co-existence of man with animals now extinct, such as the Siberian mammoth and its common associate, the woolly-haired rhinoceros. The more