In 1829 Schmerling commenced his investigations in the caves of the valley of the Meuse. For years he continued his work under many difficulties. Sir Charles Lyell tells us he was let down day after day to the opening of the Engis Cave by a rope tied to a tree. Arriving there he crawled on all fours through a narrow passage way to the enlarged chamber, where, standing in mud and water, he superintended the investigations. He examined over forty of those caves, and published his results in 1833. He clearly showed that man must have been living along with various animals now extinct in Belgium. But, as before remarked, it was deemed sufficient answer to this careful explorer to point out that his results were opposed to the accepted chronology, and so they were passed by. When the time at last came, and their true worth was recognized, Schmerling himself had passed away.
We have already seen what great results followed the exploration of DePerthes in the river gravels. When it had been clearly established that man and extinct animals hid coexisted in Europe, the results of cave explorations were eagerly recalled, and governments vied with royal societies and private individuals in continuing the researches. The results are that a rich store of facts has been gathered from those gloomy resorts, illustrative of the later stages of Paleolithic art.
A word as to the formation of caves, grottoes, caverns, and rock shelters. These vary greatly in size, some being so small as to furnish protection to but few individuals; others, especially caves, so large that whole tribes might have found a place of resort within their chambers. They are found in all limestone countries. The formation of caves is now recognized as due to natural causes acting slowly through many years. Limestone rock is very hard and durable, but chemistry teaches us that water charged with carbonic acid gas will readily dissolve it. Rain-water falling from the clouds is sure to come in contact with masses of decaying vegetable matter, which we know is constantly giving off quantities of this gas. Laden with this the water sinks into the ground, and, if it comes in contact with limestone, readily washes some of it away in solution. But beds of limestone rock are noted for containing great fissures through which subterranean waters penetrate far into the ground. We can readily see how this percolating water would dissolve and wear away the surface of the rocks along such a fissure, and in process of time we would have the phenomenon of a stream of water flowing under ground.
Owing to a great many causes—such, for instance, as the meeting of another fissure—we would expect that portions of this underground way would become enlarged to spacious halls. In some such a way as this it is now understood that all caves have originated.
Owing to many natural causes the river may, after a while, cease to flow, leaving enlarged portions of its channel behind as a succession of chambers in a cave. But water would still come trickling in from the tops and sides, and be continuously dripping to the floor, where it speedily evaporates. When such is the case it leaves behind it the limestone it held in solution. So, in process of time, if the deposition is undisturbed, there will be formed over the floor of the cave a more or less continuous layer of limestone matter known as stalagmite. The same formations on the top and sides of the cave are called stalactites. In places where the drip is continuous the stalactite gradually assumes the shape of an immense icicle; while the stalagmite on the floor of the cave, underneath the drip, rises in a columnar mass to meet the descending stalactite. A union of these is not uncommon, and, we have pillars and columns presenting the strange, fantastic appearance on which tourists delight to dwell in their notes of travel.
While these accumulations are in all cases very slow, still we can not measure the time since it commenced by the rate of present growth, because this rate varies greatly at different times and places even in the same cave. And we must also remark that this complete series of changes only occur in a few localities, the majority of caves being insignificant in size.2
From what has been said as to the formation of caves, we would expect them to occur in river valleys, and this is the case, though in some instances there have been such immense changes in the surface level of the country that we can now find no trace of rivers near them. This is exactly similar to some gravel deposits, which, as we have seen, are occasionally found where is now no running water. The most noted caverns, however, are found high up on the banks of existing rivers. We can not doubt that the rivers were the cause of the caves. But having excavated their beds below the level of the then existing caves, they ceased to flow in them, and left them to be occupied by savage animals and the scarcely less savage men. But at times, swollen by floods, the river would again assert its supremacy and roll its waters through its old channels.
These floods would not only tear up and rearrange whatever débris had already accumulated, but would introduce quantities of sediment and animal remains. In some such a manner as is here pointed out (though exactly how geologists are not agreed) caves were invaded, after being long occupied by men or animals, by floods of water. In many cases the evidence would seem to indicate that after such a visitation by water the cave and its water-rolled and water-arranged contents were left to silence, visited by neither man nor beast. In such instances stalagmitic coverings would gradually form over the confused débris, and in some places acquire a thickness of several feet. In some instances several such floors are found one above the other, pointing to a prolonged period of usage, and then a quiet stage, in which the drip of falling water alone broke the silence, and nature sealed up another chapter of cave biography beneath the layer of stalagmite.
One of the most important caves of England is Kent’s Cavern, before mentioned. This cave was carefully explored under the direction of a committee appointed by the British Association, and to show the care and thoroughness of the work we need only state that this work occupied the greater portion of sixteen years, and hence the results obtained may be regarded as, in a general way, illustrative of the life of the cave dwellers. “This cave is about a mile east of Torquay harbor, and is of a sinuous character, running deeply into a hill of Devonian limestone, about half a mile distant from the sea. In places it expands into large chambers, to which various distinctive names have been given.”3