If, then, the existing mountain chains show complex folding in their constituent rocks, and though geologically but of yesterday have been already deeply affected by the denuding agents, must we not suppose that the folded and contorted uplands of Europe and elsewhere are the last remnants of very ancient mountain chains? It is they which form the framework of the continents, and by their wear and tear the low grounds have been formed, owing to the filling in of the great gulfs which ran between the old mountain chains. We may elaborate a little further this very interesting subject. Let us first note that the geologists group the rocks composing the earth's crust into three great divisions. We have, first, the Primary rocks, which are the oldest, and include as their most generally interesting member the Carboniferous rocks, with their coal-bearing beds, so important in the modern industrial world. Second, we have the Secondary beds, the most interesting members of which is the Chalk, so well-developed in parts of England and France. Finally, the Tertiary series includes the rocks of the period immediately preceding that in which the first undoubted remains of man occur. Each of these periods was of enormous length, and the labours of successive generations of geologists have brought to light, at least in broad outline, the general appearance of the globe in so far as affected by the distribution of land and water, and the main earth movements, in each separate period. Thus we know that during that long period of time which is included in the Primary epoch, very extensive earth movements, resulting in extensive folding and mountain formation, took place. The geologists distinguish no less than three separate periods of folding in Primary times. It is not necessary for us to consider these in detail; their total result was to produce the mountain regions whose worn-down stumps now form those uplands which we have described in Europe. But they do not occur in Europe alone. That vast and relatively infertile area in Eastern Canada which geologists call the Canadian Shield is a region of very old rocks, once folded into a mountain region, but long since worn down to an upland. In the eastern United States that long, but interrupted, range of hills, which, under various names, runs from the mouth of the St. Lawrence to Alabama and Georgia, and partially shuts the seaboard off from the prairies and plains beyond, is a region where the folding is still well marked, in spite of long denudation.