The total thickness of all these rocks has been estimated at about one hundred thousand feet, or not far from twenty miles. These names have been given partly from the region in which the rocks occur, partly from the nature of the rocks themselves, and partly for other reasons. Thus the Old Red Sandstone is so called, because it generally, though not always, appears as a dark red sandstone. But the Silurian rocks, which we find in North Wales, receive their name from the Silures, an ancient Welsh tribe; the Cambrian rocks take theirs from Cambria, the old name for North Wales. The Cretaceous rocks are partly composed of chalk, for which the Latin word is _creta_; and so on. The terms "Palæozoic," "Mesozoic," and "Cainozoic" mean "ancient life," "middle life," and "recent or new life," thus indicating that as time went on the various types of life that flourished on the earth became less old-fashioned, and more like those prevailing at the present time. These used to be called "Primary," "Secondary," and "Tertiary;" but the terms were unfortunate, because the primary rocks, as then known, were not the first, or oldest. We have therefore included the Archæan rocks, since discovered, in this primary group. Only one fossil has been found in these rocks, and that is a doubtful one; hence they are sometimes called "Azoic," that is, "without life." The Mesozoic rocks are, as it were, the records of the "middle ages" in the world's history; while the Palæozoic take us back to a truly primeval time. We have now learned how the geological age of any group of rocks may be determined. Thus, if a series of rocks of unknown age can be shown to rest on undoubtedly Silurian rocks in one place, and in another place to be overlaid or covered by undoubtedly Carboniferous rocks, they will probably belong to the Old Red Sandstone Period. If afterwards we find that they contain some of the well-known fossils of that period, the question of their age is settled at once. But we want more evidence than this. Suppose, now, we find somewhere on the flanks of a mountain-range a series of Permian and Triassic rocks, resting almost horizontally on disturbed and folded Carboniferous strata. Does not that at once prove that the upheaval took place before the Permian Period? Clearly it does, because the Permian rocks have evidently _not_ been disturbed thereby. So now we can fix the date of our range of hills; namely, after the Carboniferous Period and before the Permian Period.