Valleys are formed by the separation of chains of mountains or of hills. Those which are formed between high mountains, are commonly narrow and long, as if they had originally been only fissures dividing their respective chains, or for the passage of extensive torrents. The angles of their direction sometimes exhibit singular symmetry. In the Pyrenees there are said to be valleys whose salient and re-entrant angles so perfectly correspond, that if the force which separated them were to act in a contrary direction, and bring their sides together again, they would unite so exactly that even the fissure would not be perceived. There are some highly situated valleys containing rivers and lakes which have no outlets or streams. Most high valleys have their surface upon a level with the summits of the secondary mountains in the neighborhood. The lower valleys widen as they recede from the secondary mountains from which they originate, and gradually lose themselves in the plains. Their opposite angles correspond regularly, but are very obtuse. The sort of narrow passage by which we enter into these high valleys is called a pass or defile. Between Norway and Sweden is one of these passes, formed by several masses of rock cut by nature into the shape of long parallelograms, and which have between them a passage shut in by perpendicular walls. This pass is near Skiærdal; another of the same kind is at Portfeld, or the Mountain of the Gate. These openings exactly resemble those by which the Hudson passes through successive chains of mountains, which seem desirous of checking its course. The Cordilleras of the Andes present the most stupendous passes of this kind that are known; they are from four to five thousand feet deep. The valleys of the Hudson and Connecticut are equalled by few in the old world for natural beauty and romantic scenery. Of the valleys of Europe, that of the Rhine is most celebrated; and is only more interesting than the Hudson on account of its old historical associations, its populous cities, and the picturesque ruins and massive monuments of architecture which frown upon its banks. CHAPTER III.――PRAIRIES AND PLAINS.