A mountain group, with its central peak or spire, its long ridges, steep walls, towers, buttresses, dark hollows, and carved pinnacles standing out against the sky, has well been compared to a great and stately building such as a cathedral or a temple. Mountains are indeed "a great and noble architecture, giving first shelter, comfort, and rest, but covered also with mighty sculpture and painted legend;" and to many they are Nature's shrines, where men may offer their humble praises and prayers to the great Architect who reared them for His children. We have introduced this illustration because it will help us in our inquiry. Suppose we were standing in front of some great cathedral, such as Milan, with all its marble pinnacles, or Notre Dame, with its stately towers, or the minsters of York or Durham in our own country, and trying to picture to ourselves how it was built. No one has lived long enough to watch the completion of one of these great buildings; but for all that, we know pretty well how it was made, even by watching the builder's operations for a short time, or by following, as we often may, the various stages in the construction of a small house. So it is with Nature's work. We cannot, in our little lives, witness the rearing of a great mountain-chain, or even the carving of a single hill; but we can observe for ourselves the slow and continuous operations which in the course of thousands and thousands of years produce such stupendous results. We may learn how the building operations are conducted, though the final results will only be manifested in the far-distant future.