It is by such reasoning that the age of our Pennine range of hills, extending from the north of England into Derbyshire, has been fixed; for the Permian and Triassic strata lie undisturbed on the upheaved arch of Carboniferous rocks of which this chain is composed. Its structure is that of a broken and much denuded anticline, which stands up to form a line of hills only because the Carboniferous limestone is so much harder than the "coal measures," or coal-bearing rocks, on each side of it, that it has not been worn away so fast. In time, this great anticline will be entirely worn away like that of the Weald. It is called the Great Mountain Limestone, because it so often rises up to form high ground. The Mendip Hills in Somersetshire are of about the same date, and they too are largely composed of this great limestone formation. Of course, a certain amount of up and down movement took place after the hills were upheaved, otherwise the Permian and Triassic rocks could not have been deposited on their sides; but these movements were slight and of a more general kind than those by which strata are thrown into folds. The main upheaval, by which the rocks now forming the Highlands of Scotland were lifted up and contorted, took place after the Lower Silurian Period, and before that of the Old Red Sandstone; and there is clear evidence that even before the latter period they had not only been greatly altered, or "metamorphosed," by subterranean heat, but that they had suffered enormous denudation. And the work of carving out these mountains has gone on ever since; for even in Old Red Sandstone times they were probably not entirely covered by water. The Highland Mountains are therefore older than the Pennine range. Geologically Scotland belongs in great part to Scandinavia; and the long line of Scandinavian Mountains is a continuation of the Highlands, and so is of the same age. Mountain-chains and hill-ranges have been upheaved at various geological periods; and some are very old, while others are much younger. Turning to the southeast of England, we find the ranges of chalk hills forming the North and South Downs (see page 237). As explained previously, these owe their existence to the upheaval and subsequent denudation of the low arch, or anticline, of the Weald. They are called "escarpments," because they are like lines of cliffs that are being gradually cut back. Now, it is clear that these hills are much newer than either of those we have just considered. Look at the table on page 324, and you will see that the Cretaceous rocks (chalk, etc.) belong to the Mesozoic era. The chalk was the last rock formed during the Cretaceous Period. So the Wealden arch must have been heaved up after the chalk was formed; that is, ages and ages later than the date of the Pennine range or the Scotch Highlands. From other evidences it has been shown that this anticline was heaved up in the early part of the Cainozoic Era, perhaps during the Miocene Period.