But it is difficult to determine the rate at which stalagmite accumulates. As is well known, this deposit is a form of carbonate of lime, and accumulates when water holding the substance in solution drops down upon the surface, where it is partially evaporated. It then leaves a thin film of the substance upon the floor. The rate of the accumulation will depend upon both the degree to which the water is saturated with the carbonate and upon the quantity of the water which percolates through the roof of the cavern. These factors are so variable, and so dependent upon unknown conditions in the past, that it is very difficult to estimate the result for any long period of time. Occasionally a quarter of an inch of stalagmite accretion has been known to take place in a cavern in a single year, while in Kent's Cavern, over a visitor's name inscribed in the year 1688, a film of stalagmite only a twentieth of an inch in thickness has accumulated. If, therefore, we could reckon upon a uniformity of conditions stretching indefinitely back into the past, we could determine the age of these oldest remains of man in Kent's Hole by a simple sum in arithmetic, and should infer that the upper layer of stalagmite required 240,000 years, and the lower 576,000 years, for their growth, which would carry us back more than 700,000 years, and some have not hesitated to affix as early a date as this to these lowest implement-bearing gravels. But other portions of the cave show an actual rate of accretion very much larger. Six inches of stalagmite is there found overlying some remains of Romano-Saxon times which cannot be more than 2,000 years old. Assuming this as the uniform rate, the total time required for the deposit of the stalagmitic floors would still be about 70,000 years. But, as we have seen, the present rates of deposition are probably considerably less than those which took place during the moister climate of the Glacial epoch. Still, even by supposing the rate to be increased fourfold, the age of this lower stratum would be reduced to only 12,000 years. So that, as Mr. James Geikie well maintains, "Even on the most extravagant assumption as to the former rate of stalagmitic accretion, we shall yet be compelled to admit a period of many thousands of years for the formation of the stalagmitic pavements in Kent's Cavern."[EL] We should add, however, that there is much well-founded doubt whether the implements found in the lowest stratum were really in place, since, according to Dr. Evans, "Owing to previous excavations and to the presence of burrowing animals, the remains from above and below the stalagmite have become intermingled."[EM] [Footnote EL: Prehistoric Europe, p. 83.] [Footnote EM: Stone and Flint Implements, p. 446.]