As regards the prodigious abundance of Fossil Shells preserved in the Crust of the Earth, it is unnecessary for us here to speak. We have already seen that the great mass of many limestone formations is composed almost exclusively of such remains, broken up into minute fragments, and more or less altered by chemical agency; and besides, there are quarries within the reach of all, where they may collect at pleasure these interesting relics of the olden time. But there are one or two facts of peculiar significance connected with Fossil Shells, which it may be useful briefly to set down. In the first place, we would remind our readers that there is a marked and well-known difference between the shells of those animals that can live only in the sea, of those that inhabit rivers, and of those, finally, that frequent the brackish waters of estuaries. Now it has been made clear beyond all reasonable doubt, by the explorations of Geologists, that sea-shells abound in great numbers far away from the present line of coast, in the heart of vast continents. And they are found, not merely on the surface, but buried deep in the Crust of the Earth, and overlaid, in many cases, by numerous strata of solid rock, thousands of feet in thickness. It is also to be observed that they occur at all heights above the level of the ocean; having been discovered at an elevation of eight thousand feet in the Pyrenees, ten thousand in the Alps, thirteen thousand in the Andes, and above eighteen thousand in the Himalaya.[79] Such are the phenomena which are constantly forcing themselves on the attention of the Geologist, and which involve a number of problems that he cannot help attempting to investigate and explain. He is instinctively impelled to ask himself, how can the shells of marine animals have come to exist so far away from the sea? how have they been buried in the Crust of the Earth? how have they been lifted up to the highest pinnacles of lofty mountains?