The important fact to remember about these fluviatile deposits is this—­that here the ordinary geological rule is reversed.  The high-level gravels are, of course, the highest, so far as their actual elevation above the sea is concerned; but geologically the lowest, since they are obviously much older than the low-level gravels, as these are than the recent gravels.  How much older the high-level gravels may be than the low-level ones, it is impossible to say.  They occur at heights varying from 10 to 100 feet above the present river-channels, and they are therefore older than the recent gravels by the time required by the river to dig out its own bed to this depth.  How long this period may be, our data do not enable us to determine accurately; but if we are to calculate from the observed rate of erosion of the actually existing rivers, the period between the different valley-gravels must be a very long one.

The lowest or recent fluviatile deposits which occur beside the bed of the present river, are referable to the Recent period, as they contain the remains of none but living Mammals.  The two other sets of gravels are Post-Pliocene, as they contain the bones of extinct Mammals, mixed with land and fresh-water shells of existing species.  Among the more important extinct Mammals of the low-level and high-level valley-gravels may be mentioned the Elephas antiquus, the Mammoth (Elephas primigenius), the Woolly Rhinoceros (R.  Tichorhinus), the Hippopotamus, the Cave-lion, and the Cave-bear.  Along with these are found unquestionable traces of the existence of Man, in the form of rude flint implements of undoubted human workmanship.

The so-called “Cave-deposits,” again, though exhibiting peculiarities due to the fact of their occurrence in caverns or fissures in the rocks, are in many respects essentially similar to the older valley-gravels.  Caves, in the great majority of instances, occur in limestone.  When this is not the case, it will generally be found that they occur along lines of sea-coast, or along lines which can be shown to have anciently formed the coast-line.  There are many caves, however, in the making of which it can be shown that the sea has had no hand; and these are most of the caves of limestone districts.  These owe their origin to the solvent action upon lime of water holding carbonic acid in solution.  The rain which falls upon a limestone district absorbs a certain amount of carbonic acid from the air, or from the soil.  It then percolates through the rock, generally along the lines of jointing so characteristic of limestones, and in its progress it dissolves and carries off a certain quantity of carbonate of lime.  In this way, the natural joints and fissures in the rock are widened, as can be seen at the present day in any or all limestone districts.  By a continuance of this action for a sufficient length of time, caves may ultimately be produced.  Nothing, also, is commoner in a limestone district than for the natural