Now look at the section through the Appalachian chain (see Fig. 1), and you will see that each hill is a syncline, and the valleys between them are anticlines. This happens so frequently that almost every range of mountains furnishes examples; but as every rule has its exceptions, so this one has, and we may find an example in the case of the Jura Mountains outside the Alps. It will be seen from the section that the ridges are formed by anticlines, and the valleys by synclines. But on looking a little more closely, we see that the tops of the former have suffered a considerable amount of erosion (as indicated by the dotted lines). Now, the reason why they have not been completely worn down into valleys is that these rocks were once covered by others overlying them, so that this outer covering of rocks had first to be removed before they could be attacked by rain and rivers. These wave-like ridges of the Jura are being slowly worn down; and the time must come when they will be carved out into valleys, while the synclines between them will stand out as hills. It is simply a question of time. But many mountain-chains have a far more complicated structure than that of the Appalachians, and consist of violently crumpled and folded strata (see section of Mont Blanc, Fig. 3). [Illustration: SECTIONS OF MOUNTAIN-RANGES, SHOWING THEIR STRUCTURE AND THE AMOUNT OF ROCK WORN AWAY.] It might naturally be asked how such sections are made, considering that we cannot cut through mountains in order to find out their structure; but Nature cuts them up for us, gashing their sides with ravines and valleys carved out by streams and rivers, and in steep cliffs and precipices we find great natural sections that serve our purpose almost equally well. Sometimes, however, we get considerable help from quarries and railway-cuttings. Take, for example, one of the synclinal folds in the Appalachian chain. Its structure is ascertained somewhat as follows. Suppose you began to ascend the hill, armed with a good map, a pocket-compass, a clinometer,--a little instrument for measuring the angles at which strata dip or slope,--and with a bag on your back for specimens of rocks and fossils. At the base of the hill you might notice at starting a certain layer of rock--say a limestone--exposed by the side of the stream. It will be so many feet thick, and will contain such-and-such fossils, by means of which you can identify it; and it will dip into the interior of the hill at a certain angle, as measured by the clinometer. As you rise higher, this rock may be succeeded by sandstone of a certain thickness, and likewise dipping into the hill; and so with the other rocks that follow, until you reach the summit.