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.
By the time you have reached the top of the hill, you know the nature of all the rocks up that side, and the way they dip; and all your observations are carefully recorded in a notebook. Then you begin to descend on the other side, and in so doing you find the same set of rocks coming out at the surface all in the same order; only this order is now reversed, because you are following them downwards instead of upwards. Of course they are hidden in many places by soil and loose stones; but that does not matter, because at other places they are exposed to view, especially along ravines, carved out of the mountain-side. Also rocks "weather" so differently that they can often be distinguished even at a distance.
In this kind of way you can find out the structure of a mountain, and draw a section of it when you get home, by following out and completing the curves of the strata as indicated at or near the surface; and you find they fit in nicely together.
Fig. 3 (see page [307]) represents what is believed to be the general arrangement of the rocks of Mont Blanc. The section is greatly simplified, because many minor folds and all the faults, or dislocations, are omitted. Now, in this case we have an example of what is known as the "fan-structure." It will be seen at once that the folds have been considerably squeezed together; and the big fold in the centre indicated by dotted lines has been so much compressed in the lower part—that is, in what is now Mont Blanc—that its sides were brought near to each other until they actually sloped inwards instead of outwards.
You may easily imitate this structure by taking a sheet of paper, laying it on the table, and then, putting one hand on each side of it, cause it to rise up in a central fold by pressing your hands towards each other. Notice carefully what happens. First, you get a low arch, or anticline, like that of the Weald. Then as you press it more, the upward fold becomes sharper and narrower; then continue pressing it, and you will find the fold bulging out at the top, but narrowing in below until you get this fan-structure.
This is just what has happened in the case of the Alps. A tremendous lateral pressure applied to the rocks heaved them up and down into great and small folds, and in some places, as in Mont Blanc, fan-structure was produced. Imagine the top of the fan removed, and you get what looks like a syncline, but is really the lower part of a very much compressed anticline.